Detection of gas-phase analytes using liquid crystals

ABSTRACT

Provided herein is technology relating to detecting gaseous analytes and particularly, but not exclusively, to devices and methods related to detecting gaseous analytes by monitoring changes in liquid crystals upon exposure to the gaseous analytes.

This application is a continuation of U.S. patent application Ser. No.14/774,964, filed Sep. 11, 2015, which is a 371 U.S. National PhaseEntry of International Application No. PCT/US2014/024735, filed Mar. 12,2014, which claims priority to U.S. Prov. Pat. Appl. Ser. No.61/779,561, filed Mar. 13, 2013, and U.S. Prov. Pat. Appl. Ser. No.61/779,569, filed Mar. 13, 2013, the contents of which are incorporatedherein by reference in their entireties.

FIELD OF TECHNOLOGY

Provided herein is technology relating to detecting volatile organiccompounds and gaseous analytes and particularly, but not exclusively, todevices and methods related to detecting volatile organic compounds andgaseous analytes by measuring changes in the physical properties ofliquid crystals upon exposure to the volatile organic compounds andgaseous analytes.

BACKGROUND

The types and concentrations of synthetic chemicals that are in ourenvironment are of greater concern to government, businesses, andsociety in general than in any time in history. Multiple factorscontribute to this heightened concern, such as national security issuesrelated to the use of deadly chemicals as weapons, the risk of anintentional or accidental chemical spill, environmental awareness, andincreased understanding of the potential impacts of such chemicals onhuman health. The range of applications for sensors that can accuratelymeasure volatile gases is wide.

For example, the Department of Homeland Security needs sensors to detectthe presence of chemical weapons, such as chemical warfare agents andexplosives. These sensors can be integrated into traffic lights in largecities, as components of air-intake valves in municipal buildings, andused as on-board devices for unmanned aerial vehicles or roboticvehicles that are used to explore hazardous situations. Similar sensorscan be used to detect natural gas leaks for home and business owners andto monitor outdoor air in local communities, school playgrounds, oragricultural settings.

Approximately 70,000 illnesses and deaths occur annually as a result ofoccupational exposure to toxic gases, at a cost of more than $100billion from lost wages and medical expenses Millions of US workers invarious industries are exposed to vapors from various organic chemicalsthat are recognized by the National Institute of Occupational Safety andHygiene (NIOSH) as carcinogens, reproductive hazards, and/orneurotoxins. As such, industrial manufacturers need sensors to monitorfacility air during production, survey product off-gassing, and assistwith maintaining safe levels of permissible exposure limits (PELs) toprotect workers against the health effects of exposure to hazardoussubstances including toxic industrial chemicals. NIOSH and othergovernmental agencies such as the Environmental Protection Agency,Occupational Safety and Health Agency, Housing and Urban Development,and the Federal Emergency Management Agency are tasked with reducing therisk, and therefore the healthcare burden, of exposure to toxic gases,while attempting to minimize the impact on industry operations andrevenues.

For example, volatile organic compounds (VOCs) are a class of widelyused organic chemicals that present significant long-term and short-termhealth risks. These compounds have a high vapor pressure in ambientconditions and thus are readily outgassed from products that containthem. For example, VOCs are present in a wide array of products such aspaints and lacquers, paint strippers, cleaning supplies, pesticides,building materials and furnishings, office equipment such as copiers andprinters, correction fluids and carbonless copy paper, graphics andcraft materials including glues and adhesives, permanent markers, andphotographic solutions. Consequently, many people are exposed to VOCsdaily.

To protect these people from dangerous exposure to hazardousenvironments, there is a need for inexpensive devices that measure theconcentration of these harmful compounds. Existing devices formeasurement of VOCs rely mainly on the photoionization detector-basedtechnology and require high power to operate and are expensive for wideapplications. The devices based on colorimetric detection of VOCs, onthe other hand, are ambiguous and do not provide quantitativemeasurement. Therefore, there is an unmet need for a simple technologythat enables development of an inexpensive sensor device forquantitative detection of VOCs for a number of applications includingHazMat/Homeland Security, industrial hygiene, indoor air quality,military applications, and biomedical applications.

In addition to applications related to monitoring exposure to dangerousgas-phase chemicals and protecting the health of individuals, detectionof gas-phase analytes finds uses in industrial and commercial settings.For example, some industrial applications include monitoring productperformance such as interrogating vehicle emissions for release ofvolatile gases. Additional applications include assessing fruit ripenessand/or spoilage based on volatile gas emissions.

There is also broad potential for use of sensors of gas-phase analytesin biomedical applications. For example, sensors have been used tomonitor the composition of gas mixtures used for anesthesia duringsurgical procedures or to monitor exhaled gases related to metabolicactivities. Recently, analysis of human breath has emerged as anon-invasive technique for diagnosis of disease. The exhaled humanbreath contains a number of volatile gases such as oxygen, carbondioxide, nitrogen, carbon monoxide, acetone, ammonia, hydrogen sulfide,amines, oxides of nitrogen, etc. (Manolis, 1983; Smith et al, 1999; andDiskin et al, 2003) and measurements of analytes in exhaled breath havebeen applied to a wide range of disease states, including diabetes(Henderson et al, 1952; Sulway et al, 1970; Crofford et al, 1977; andNovak et al, 2007), gastrointestinal disorders (Perman, 1991; Bauer etal, 2000; and Nieminen et al, 2000), and asthma (Alving et al, 1993).

While current technologies exist to measure gaseous analytes (e.g.,volatile organic compounds and other compounds), these technologies donot provide timely information regarding gas levels to inform immediateactions for minimizing risks, e.g., taking appropriate measures in themedical, defense, and industrial settings to protect human health. Forexample, many direct-read dosimeters lack sensitivity andreproducibility and do not meet regulatory monitoring requirements.Alternatively, many indirect read technologies are shipped to anaccredited laboratory for analysis, introducing a long lag-time oftypically many weeks between sample collection and data retrieval. Inaddition, conventional technologies are also subject to substantialpositive or negative interference from other pollutants and inaccuraciesresulting from low air flow. There exists, therefore, an unmet need fortechnology that accurately measures gases and that can be read on-siteto provide actionable information. In some situations described above,it is necessary to know the concentration of the chemical environment asquickly as possible in order to minimize exposure to the chemical. Insuch situations a detector of the instantaneous concentration of the gasis needed. In other situations, such as but not limit to when measuringpersonal exposure to a vapor, it is necessary to know the cumulativeexposure to the chemical that occurs of a given interval of time, suchas but not limited to a workday. In this situation, a dosimeter thatmeasures the cumulative level of exposure over a set period of time isneeded.

SUMMARY

Provided herein is technology related to sensors and analyte detectionassociated with the use of liquid crystal (LC) materials. In someembodiments, the technology relates to detecting volatile organiccompounds and particularly, but not exclusively, to methods andcompositions for detecting volatile organic compounds by measuringchanges in the physical properties of liquid crystals. In someembodiments, the LC materials that find use in the technology compriserod-shaped organic molecules that form condensed phases. These materialspossess long-range orientational ordering (and are thus in some aspectscrystal-like) but lack positional ordering (and are thus in some aspectsliquid-like). The long-range ordering of molecules within the LC givesrise to anisotropic optical properties and optical birefringence. Theinteraction of gas-phase analytes with particular LC materials and orwith the surface supporting it modifies the long-range order ororientation of the LCs and produces distinct changes in the opticalappearance of the LC, thus providing a measureable indicator associatedwith the analyte.

Accordingly, some embodiments of the technology provide methods fordetecting an analyte (e.g., a VOC) in a gaseous phase, the methodcomprising providing a liquid crystal assay device; exposing the liquidcrystal assay device to a sample suspected of comprising an analyte; andinterrogating the liquid crystal assay device to detect the analyte,wherein a change in a property of the liquid crystal composition in theliquid crystal assay device caused by an interaction of the analyte withthe liquid crystal assay device is indicative of the presence of theanalyte. In some embodiments the liquid crystal assay device comprises afirst surface contacting a composition comprising a liquid crystal; asecond surface; and a headspace between the composition comprising theliquid crystal and the second surface. In some embodiments, the firstsurface comprises a functional group and an interaction of the analytewith the functional group causes the change in the property of theliquid crystal composition. For example, some embodiments provide thatthe functional group is specific for the analyte. In some embodiments,detection of the analyte is cumulative. In some embodiments, the changein a property of the liquid crystal composition is detectable inreal-time. In some embodiments, the presence of said analyte is detectedin real-time. The headspace is related to the rate of exposure of thedevice to the analyte and, as such, provides a functionality to controlthe rate of exposure of the device to the analyte. In some embodiments,the headspace is 1 to 100 microns, 5 to 50 microns, or, in someembodiments, 10 to 25 microns. In some embodiments, the headspace isvariable. In some embodiments, the first and second surfaces form acompartment having first and second open ends, wherein the headspace atthe first end is from 1 to 20 microns and the headspace at the secondend is from 21 to 100 microns.

The thickness of the liquid crystal film is related to the response ofthe device to the analyte, e.g., by controlling the rate at which theanalyte reaches a functionalized surface upon which the liquid crystalhas been deposited. Embodiments are provided in which micro-pillarfeatures on a surface control the thickness of the liquid crystal film.Accordingly, in some embodiments the first surface further comprisesmicro-pillars. The surfaces are not limited in the materials from whichthey are made. For instance, some embodiments provide that the firstsurface and/or the second surface comprises a substrate of glass,silicon, or gold. In some embodiments, the second surface isfunctionalized with an intert substance. In some embodiments, the secondsurface is functionalized with(Tridecafluoro-1,1,2,2-tetrahydrooctyl)-trichlorosilane.

Some embodiments provide a device in which the analyte interactsdirectly with the liquid crystal to effect a phase in the liquidcrystal. Some embodiments provide a device in which the liquid crystalcontacts a surface (e.g., a functionalized surface) and the interactionof the analyte with the functionalized surface effects an orientation ofthe liquid crystal. In some embodiments, the functionalized surfacecomprises a functional group that is 4-aminothiophenol. In someembodiments, the functionalized surface comprises a functional groupthat is lead perchlorate.

The technology is not limited in the liquid crystal that is used toindicate the presence of the analyte. For example, some exemplary liquidcrystals that find use in the technology are MBBA, MLC-2080, MLC-2081,and E7, and mixtures thereof. Various liquid crystal compositions finduse in embodiments of the technology. For instance, in some embodimentsthe liquid crystal composition comprises a cyanobiphenyl compound.

Particular exemplary embodiments of the technology detect the analytesH₂S, HCHO, NO₂, toluene, benzene, xylene, nitrobenzene, hexane, alcohol,gasoline, and components of gasoline (e.g., octane, etc.).

A phase change in a liquid crystal can be associated with a change inthe optical anisotropy, magnetic anisotropy, dielectric anisotropy,and/or the presence of a phase transition temperature. Accordingly, someembodiments provide methods wherein the interrogation comprisesmeasuring a change in a property selected from the group consisting ofoptical appearance, optical anisotropy, magnetic anisotropy, dielectricanisotropy, rheology, optical absorbance, and phase transitiontemperature. In some embodiments, exposing the liquid crystal assaydevice to a sample suspected of comprising an analyte causes a phasetransition in the liquid crystal composition from a first phase selectedfrom the group consisting of an isotropic phase, a nematic phase, or asmectic phase to a second phase selected from the group consisting of anisotropic phase, a nematic phase, and a smectic phase. In someembodiments, the liquid crystal composition undergoes an orientationaltransition in the presence of the analyte, such as a change in theorientation of the optical axis of the liquid crystal (e.g., a change inthe tilt of the liquid crystal from the surface normal). In someexemplary embodiments, the orientational transition is selected from thegroup consisting of a homeotropic alignment changing to a planaralignment, a random planar alignment changing to a uniform planaralignment, a uniform planar alignment changing to a random planaralignment, and a planar alignment changing to a homeotropic alignment.

Embodiments are provided in which a headspace provides a channel foraccess of the analyte to the device and liquid crystal. As such, someembodiments provide that the second surface does not contact thecomposition comprising the liquid crystal.

In some embodiments, the concentration or accumulated exposure to ananalyte is related to the size of an area of the device in which theliquid crystal has undergone a phase change (a “reacted area” of thedevice). Consequently, embodiments are provided in which method comprisequantifying an analyte concentration by measuring a size of a LCresponded area. For instance, in some embodiments the methods comprisequantifying an analyte concentration by measuring a distance of abirefringent front from a site of exposure of the liquid crystal assaydevice to the sample suspected of comprising the analyte. In someembodiments, measuring an anisotropy provides an observable property todifferentiate the phases (e.g., the “unreacted area” and the “reactedarea”) and thus assess the size (e.g., the length) of the reacted area.In some embodiments, the anisotropy is an optical anisotropy and theinterrogation comprises measuring a reflection or a transmission ofpolarized light. In some embodiments, measurement of the rate ofincrease in the reacted area is used to indicate the concentration ofthe analyte present around the device.

In some embodiments, methods are provided detecting an analyte in agaseous phase, the methods comprising providing a liquid crystal assaydevice comprising a surface in a channel; exposing the liquid crystalassay device to a sample suspected of comprising an analyte; contactingthe surface with a liquid crystal; and interrogating the liquid crystalassay device to detect the analyte, wherein a change in a property ofthe liquid crystal composition in the liquid crystal assay device causedby an interaction of the analyte with the liquid crystal assay device isindicative of the presence of the analyte. In these embodiments, thesurface is reacted with the analyte and the liquid crystal is applied to“read” the reacted portion of the surface. For instance, in someembodiments the surface comprises a functional group and an interactionof the analyte with the functional group causes the change in theproperty of the liquid crystal composition. Particular embodimentsprovide that the functional group is specific for the analyte.

The surfaces are not limited in the materials from which they are made.For instance, some embodiments provide that the first surface and/or thesecond surface comprises a substrate of glass, silicon, or gold.

Some embodiments provide a device in which the liquid crystal contacts asurface (e.g., a functionalized surface) and the interaction of theanalyte with the functionalized surface effects a phase change in theliquid crystal. In some embodiments, the functionalized surfacecomprises a functional group that is 4-aminothiophenol. In someembodiments, the functionalized surface comprises a functional groupthat is lead perchlorate.

A phase change in a liquid crystal causes a change in a compositioncomprising the liquid crystal such as a change in the opticalanisotropy, magnetic anisotropy, dielectric anisotropy, and/or phasetransition temperature. Accordingly, some embodiments provide methodswherein the interrogation comprises measuring a change in a propertyselected from the group consisting of optical anisotropy, magneticanisotropy, dielectric anisotropy, and phase transition temperature. Insome embodiments, exposing the liquid crystal assay device to a samplesuspected of comprising an analyte causes a phase transition in theliquid crystal composition from a first phase selected from the groupconsisting of an isotropic phase, a nematic phase, a liquid crystalphase rich in chiral dopants, a frustrated phase, a blue phase, aferroelectric phase, a twisted grain boundary phase, or a smectic phaseto a second phase selected from the group consisting of an isotropicphase, a nematic phase, and a smectic phase, a liquid crystal phase richin chiral dopants, a frustrated phase, a blue phase, a ferroelectricphase, a twisted grain boundary phase. In some embodiments, the liquidcrystal composition undergoes an orientational transition in thepresence of the analyte, wherein the orientational is a change in theorientation of the optical axis of the liquid crystal. In some exemplaryembodiments, the orientational change is selected from the groupconsisting of a homeotropic alignment changing to a planar alignment, arandom planar alignment changing to a uniform planar alignment, auniform planar alignment changing to a random planar alignment, and aplanar alignment changing to a homeotropic alignment. In otherembodiments, the orientational transition involves a change in the tiltof the liquid crystal away from the surface normal.

In some embodiments, the concentration or accumulated exposure to ananalyte is related to the size of an area of the device in which theliquid crystal has undergone a phase or orientational change (a “reactedarea” of the device). Consequently, embodiments are provided in whichmethod comprise quantifying an analyte concentration by measuring a sizeof a reacted area. For instance, in some embodiments the methodscomprise quantifying an analyte concentration by measuring a distance ofa birefringent front from a site of exposure of the liquid crystal assaydevice to the sample suspected of comprising the analyte. In someembodiments, measuring an anisotropy provides an observable property todifferentiate the phases (e.g., the “unreacted area” and the “reactedarea”) and thus assess the size (e.g., the length) of the reacted area.In some embodiments, the anisotropy is an optical anisotropy and theinterrogation comprises measuring a reflection or a transmission ofpolarized light.

In some embodiments, the size of the reacted area is on the order of 1,10, 100, or 1000 mm². For example, in some embodiments the distancemeasured for a reacted area is from about 1 micron about 200 mm, forexample, from about 1 micron to 1 mm, 1 micron to 10 mm, 1 micron to 50mm, 1 micron to 100 mm, 1 micron to 200 mm, 1 mm to 10 mm, 1 mm to 50mm, 1 mm to 100 mm, 1 mm to 200 mm, 10 mm to 100 mm or 10 mm to 200 mm.

The technology finds use in monitoring exposure (e.g., of a person) to agaseous analyte such as a toxic gas. Accordingly, embodiments of methodsare provided for monitoring a subject's exposure to a toxic gas, themethods comprising providing to the subject a dosimeter badge comprisinga liquid crystal assay device; measuring a change in a property of aliquid crystal composition in the liquid crystal assay device caused byan interaction of the toxic gas with the liquid crystal composition; andreporting an exposure to the toxic gas. Monitoring methods comprise useof embodiments of devices provided herein. In some embodiments thedevices report exposure in real-time to provide an immediate signal ofexposure. In some embodiments, the devices report a cumulative exposureover an amount of time (e.g., 1 to 100 minutes; 1 to 10 days; 1 to 10weeks; or more). In some embodiments, detection of the analyte is inreal-time. Embodiments comprise methods relate to a liquid crystal assaydevice that comprises a first surface contacting a compositioncomprising a liquid crystal; a second surface; and a headspace betweenthe composition comprising the liquid crystal and the second surface.Further embodiments are provided wherein the liquid crystal assay devicecomprises a surface in a channel and the method further comprisescontacting the surface with a liquid crystal.

In some embodiments, the devices are distinguishable from otherdosimeter devices, such as electrochemical devices, based on the mass ofthe device. In some preferred embodiments, the devices of the presentinvention have a mass of from about 5 to 50 grams, preferably from about10 to 30 grams or from 10 to 20 grams.

In some embodiments, the present invention provides a sensor devicecomprising a first substrate having a surface modified with an aminemoiety, said surface having disposed thereon a liquid crystalcomposition that is homeotropically aligned in the presence of the aminemoiety. In some embodiments, the substrate comprises a gold filmdisposed on an underlying base substrate. In some embodiments, thesubstrate further comprises an intervening layer between the basesubstrate and said gold film. In some embodiments, the intervening layeris a metallic adhesion layer selected from the group consisting oftitanium and chromium. In some embodiments, the amine moiety is4-aminothiophenol. In some embodiments, the substrate is a glasssubstrate. In some embodiments, the amine moiety isp-aminophenyltrimethoxysilane. In some embodiments, the liquid crystalcomposition comprises MBBA. In some embodiments, the sensor devicesfurther comprise a second substrate oriented opposite of the firstsubstrate to define a compartment. In some embodiments, the compartmenthas a headspace between the liquid crystal composition disposed on saidfirst substrate and said second substrate. In some embodiments, theheadspace is 1 to 100 microns. In some embodiments, the headspace is 5to 50 microns. In some embodiments, the headspace is 10 to 25 microns.In some embodiments, the headspace is variable. In some embodiments, thefirst and second surfaces form a compartment having first and secondopen ends, wherein the headspace at the first end is from 1 to 20microns and the headspace at the second end is from 21 to 100 microns.

In some embodiments, the technology provides a method for detecting avolatile organic compound in a gaseous phase, the method comprisingproviding a liquid crystal assay device comprising a surface in contactwith a liquid crystal composition; exposing the liquid crystal assaydevice to a sample suspected of comprising a volatile organic compound;and interrogating the liquid crystal assay device to detect the volatileorganic compound, wherein a change in a property of the liquid crystalcomposition in the liquid crystal assay device caused by an interactionof the volatile organic compound with the liquid crystal assay device isindicative of the presence of the volatile organic compound. In someembodiments, the surface comprises a substrate of glass, gold, orsilicon and in some embodiments the surface comprises micro-pillars.

Various liquid crystal compositions find use in embodiments of thetechnology. For instance, in some embodiments the liquid crystalcomposition comprises a cyanobiphenyl compound. Furthermore, thetechnology is directed toward detecting various volatile organiccompounds, e.g., in some embodiments the volatile organic compound istoluene, benzene, xylene, nitrobenzene, hexane, or an alcohol. In otherembodiments, the volatile organic compound is octane, gasoline, painerthinner or stove alcohol.

In some embodiments, the surface is a polymer deposited on a substrate.The technology is not limited in the polymer that is deposited. Forexample, in some embodiments the polymer is a polystyrene, a polyvinylacetate, or a fluoroalcohol polycarbosilane. Furthermore, in someembodiments the polymer is mechanically rubbed.

In some embodiments the surface is a hydrocarbon film deposited on asubstrate. For example, some embodiments provide that the hydrocarbonfilm comprises a long chain aliphatic hydrocarbon, for instance in someembodiments a hydrocarbon film that is solid at room temperature and issoluble in the volatile organic compound or is plasticized by thevolatile organic compound. In some embodiments the hydrocarbon comprisesan aliphatic chain of 18 or more carbons.

The methods detect volatile organic compounds over a range ofconcentrations. Exemplary embodiments are provided wherein the methoddetects a volatile organic compound of at least approximately 50 ppm.

In some embodiments, the liquid crystal composition is utilized in theform of polymer dispersed liquid crystal and in some particularembodiments the polymer dispersed liquid crystal is formed in the liquidcrystal assay device in a strained configuration.

In some embodiments, the surface comprises a cavity and the liquidcrystal composition is confined in the cavity.

In some embodiments the liquid crystal composition is confined in apolymer matrix. In some embodiments, the liquid crystal composition is apolymer dispersed liquid crystal deposited on a rubbed polymer film. Insome embodiments, the polymer dispersed liquid crystal comprisesdroplets having a diameter of less than 2 microns.

In some embodiments, the surface comprises an ionic salt. For example,in some embodiments the surface comprises a first ionic salt that issoluble in the volatile organic compound and a second ionic salt that isnot soluble in the volatile organic compound. Exemplary ionic saltsinclude a quaternary ammonium, a tetraphenylborate, or a metalperchlorate salt.

In some embodiments, the liquid crystal comprises a polymer bead.

Volatile organic compounds induce changes in a variety of physicalcharacteristics of the liquid crystal compositions. For example, in someembodiments exposing the liquid crystal assay device to a samplesuspected of comprising a volatile organic compound causes a phasetransition in the liquid crystal composition. In some embodimentsexposing the liquid crystal assay device to a sample suspected ofcomprising a volatile organic compound causes a structural change of theliquid crystal composition confined in a microstructure. In someembodiments exposing the liquid crystal assay device to a samplesuspected of comprising a volatile organic compound causes a dewettingof a polymer film on the substrate. In some embodiments exposing theliquid crystal assay device to a sample suspected of comprising avolatile organic compound causes a structural change in a polymer filmsupporting the liquid crystal composition. In some embodiments exposingthe liquid crystal assay device to a sample suspected of comprising avolatile organic compound causes dissolution of an ionic salt into theliquid crystal composition. In some embodiments exposing the liquidcrystal assay device to a sample suspected of comprising a volatileorganic compound causes swelling of a polymer bead suspended in theliquid crystal composition.

Changes in physical characteristics of the liquid crystal compositionare monitored or interrogated by various methods. In some embodimentsthe interrogation comprises measuring a change in a property such asoptical anisotropy, magnetic anisotropy, rheology, optical absorbance,dielectric anisotropy, or phase transition temperature. In someembodiments the change in the property of the liquid crystal compositionin the liquid crystal assay device caused by interaction of the volatileorganic compound with the liquid crystal assay device is a change intransmission of polarized light.

In some embodiments the liquid crystal assay device comprises an arrayof discrete assay areas and an internal calibration area and wherein theinterrogation comprises comparing the response of an assay area to theinternal calibration area.

In some embodiments the surface has a form selected from the groupconsisting of planar, spherical, and cylindrical. In some embodimentsthe surface is a patterned surface, for example, a patterned surfacethat comprises a feature such as a grid, a channel, a pillar, or anassay area, or a combination thereof. In some embodiments, the featuresare 1 to 50 microns high, 1 to 200 microns wide, and spaced 1 to 200micron apart. Moreover, in some embodiments the pillars have a form thatis circular, triangular, square, or hexagonal.

In some embodiments, exposing the liquid crystal assay device to asample suspected of comprising a volatile organic compound causes aphase transition in the liquid crystal composition from a first phasethat is an isotropic phase, a nematic phase, or a smectic phase to asecond phase that is an isotropic phase, a nematic phase, and a smecticphase. In some embodiments the liquid crystal composition undergoes anorientational transition in the presence of the volatile organiccompound, wherein the orientational transition is a homeotropicalignment changing to a planar alignment, a random planar alignmentchanging to a uniform planar alignment, a uniform planar alignmentchanging to a random planar alignment, or a planar alignment changing toa homeotropic alignment. In some embodiments, the liquid crystalundergoes an orientational transition that changes the tilt of theliquid crystal from the normal. In some embodiments the liquid crystalcomposition comprises a dopant.

Additional embodiments will be apparent to persons skilled in therelevant art based on the teachings contained herein.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presenttechnology will become better understood with regard to the followingdrawings:

FIG. 1A-C shows images of 2×5 sensors before exposure to H₂S (FIG. 1A),long sensors #1 to #4 before exposure to H₂S (FIG. 1B), and sandwichcells comprising mylar of 25 microns and 50 microns before exposure toH₂S (FIG. 1C). The bright stripe at the middle of the FIG. 1A right sideis duel to the mylar strip.

FIG. 2A-C shows images of 2×5 sensors after exposure to 1 ppm H₂S (FIG.2A), long sensors #1 to #4 after exposure to 1 ppm H₂S (FIG. 2B), andsandwich cells comprising mylar of 25 microns and 50 microns afterexposure to 1 ppm H₂S (FIG. 2C).

FIG. 3 shows an image of the long sensor #3 from FIGS. 1 and 2 afterstorage at ambient conditions for several days after exposure.

FIG. 4A-B shows sandwich cells prior to exposure to analyte (FIG. 4A) ora zero air control (FIG. 4B).

FIG. 5A-B shows images of microfluidic cells prior to exposure toanalyte (FIG. 5A) or a zero air control (FIG. 5B).

FIG. 6A-B shows images of sandwich cells after exposure to 8 ppm H₂S/45%relative humidity for eight hours (FIG. 6A) and to zero air control at45% relative humidity for eight hours (FIG. 6B).

FIG. 7A-B shows images of microfluidic cells after exposure to 8 ppmH₂S/45% relative humidity for eight hours (FIG. 7A) and to zero aircontrol at 45% relative humidity for eight hours (FIG. 7B).

FIG. 8A-J is a series of images acquired of cumulative analyte sensors.FIG. 8A shows a sensor exposed to H₂S for 0 ppm-hour. FIG. 8B shows asensor exposed to H₂S for 0.8 ppm-hour. FIG. 8C shows a sensor exposedto H₂S for 4 ppm-hour. FIG. 8D shows a sensor exposed to H₂S for 8ppm-hour. FIG. 8E shows a sensor exposed to H₂S for 25 ppm-hour. FIG. 8Fshows a sensor exposed to H₂S for 40 ppm-hour. FIG. 8G shows a sensorexposed to H₂S for 80 ppm-hour. FIG. 8H shows a sensor exposed to H₂Sfor 120 ppm-hour. FIG. 81 shows a sensor exposed to H₂S for 160ppm-hour. FIG. 8J shows a sensor exposed to H₂S for 16 ppm-hour verifyquality control.

FIG. 9 is a plot showing the length of the response front versus H₂Sexposure dose for an H₂S sensor.

FIG. 10 is a plot showing the length of the response front versus thesquare root of the H₂S exposure dose for an H₂S sensor.

FIG. 11 shows the effect of thickness of the headspace height on theresponse of microfluidic sensor.

FIG. 12 shows the effect of different concentrations of H₂S on responsefrom microfluidic sensor.

FIG. 13 shows the response from a microfluidic sensor with 45 micronhead space and with micropillared area extending to the edge.

FIG. 14 shows variation of the response length as a function of theexposure time for real-time detection of H₂S using microfluidic cell.

FIG. 15 shows the optical response of a liquid crystal sensor exposed to17.5 ppm HCHO and non-targeted vapors. The transmitted light intensitywas captured by digital camera and expressed as brightness.

FIG. 16A-C shows the detection of cumulative exposure to HCHO usingliquid crystal-based dosimeter badges. FIG. 16a shows a depiction offormaldehyde vapor diffusing into the headspace and into the LC film,thus generating a lateral concentration gradient that is seen as a darkfront on each side of the badge (FIG. 16b ). FIG. 16c is a plot showingthe measured light intensity decreasing linearly with exposure time.

FIG. 17 shows Fourier transform infrared spectra acquired of a surfacecomprising an 4-aminothiophenol film before (spectrum A) and after(spectrum B) exposure to NO₂.

FIG. 18 shows a Fourier transform infrared spectrum acquired of a 100 Åsilicon wafer comprising a 4-aminothiophenol film.

FIG. 19 shows Fourier transform infrared spectra acquired of a 100 Ågold surface comprising an 4-aminothiophenol film before (spectrum A)and after (spectrum B) exposure to NO₂.

FIG. 20A-B shows the alignment of different liquid crystals on surfacesfunctionalized with 4-aminothiophenol before (FIG. 20A) and afterexposure to NO₂ (FIG. 20B).

FIG. 21 shows the selective detection of NO₂ relative to humid gas.

FIG. 22 shows Fourier transform infrared spectra acquired of a surfacecomprising a 4-aminothiophenol film before (grey) and after (black)exposure to N₂ (top spectra) and NO₂ (bottom spectra).

FIG. 23 shows the detection of NO₂ using channels defined by apolydimethylsiloxane channel. The images were acquired 2 minutes afterfilling the LC cell

FIG. 24 shows an embodiment of a sensing device comprising apolydimethylsiloxane channel used to define the exposure path on afunctionalized surface to increase the sensitivity of detection.

FIG. 25 shows the detection of different concentrations of NO₂ usingpolydimethylsiloxane channels to define the exposure path.

FIG. 26A-B shows a series of optical images demonstrating the responseof an embodiment of the technology to different concentrations of NO₂(FIG. 26A). FIG. 26B shows a linear relationship between the length ofthe bright channel and the NO₂ concentration.

FIG. 27A-C shows the alignment of three liquid crystals having anegative dielectric anisotropy. FIG. 27A, FIG. 27B, and FIG. 27C showsthe alignment of the liquid crystals MBBA, MLC-2080, and MLC-2081,respectively.

FIG. 28A-H shows the alignment of mixtures of MBBA and MLC-2080 atdifferent ratios of MBBA to MLC-2080. Panels (a) through (h) show thealignment of MLC-2080 alone; mixtures of MLC-2080 and MBBA at ratios of0.9 MLC-2080, 0.8 MLC-2080, 0.7 MLC-2080, 0.6 MLC-2080, 0.5 MLC-2080,and 0.25 MLC-2080; and MBBA alone, respectively.

FIG. 29A-C shows on optical image of a liquid crystal cell fabricatedwith a 4-aminothiophenol functionalized surface exposed to NO₂ fordifferent times and filled with liquid crystal compositions. Panels (a)through (c) are images of a cell exposed to 20 ppb humid NO₂ for 10minutes then filled with MBBA, a cell exposed to 20 ppb humid NO₂ for 5minutes then filled with MBBA, and a cell exposed to 20 ppb humid NO₂for 5 minutes then filled with a mixture of MLC-2080 and MBBA at a 60 to40 ratio.

FIG. 30A-F shows optical images of cells comprising 4-aminothiophenolfunctionalized surfaces. Panels (a) through (f) show cells exposed to 20ppb humid NO₂ at 800 sccm for 2 minutes and filled with pure MBBA; 10ppb humid NO₂ at 800 sccm for 2 minutes and filled with MLC-2080 andMBBA at a 60 to 40 ratio; (c) 10 ppb humid NO₂ at 400 sccm for 2 minutesand filled with MLC-2080 and MBBA at a 60 to 40 ratio; (d) humid N₂ at400 sccm for 2 minutes and filled with MLC-2080 and MBBA at a 60 to 40ratio; (e) 20 ppb humid NO₂ at 800 sccm for 30 seconds and filled withMLC-2080 and MBBA at a 60 to 40 ratio; and (f) image of (e) afterovernight storage at room temperature.

FIG. 31 shows images of a LC cell comprising a 4-aminothiolphenoltreated gold surface exposed to different concentrations of NO₂ andfilled with a liquid crystal (MLC-2080 and MBBA at a 65 to 35 ratio).Concentrations of NO₂ tested were from 10 to 40 ppb in 5 ppb incrementsand 40 to 100 ppb in 10 ppb increments.

FIG. 32 shows a plot of data collected from analyzing the series ofimages shown in FIG. 31.

FIG. 33A-B is schematic showing a principle of detection of organicvapors using liquid crystals (LCs) using phase transition. The initialand post exposure appearance of the LC sensor depends on the surface theLC is laid onto. (A) A sensor consists of a thin film of LC supported ona substrate with polymeric micropillars and (B) initially appears bright(left). When the sensor is exposed to analyte (e.g., toluene), the LCmaterial turns into an isotropic material and appears dark (right)between crossed polarizers.

FIG. 34A-B is a schematic showing a principle of detection of VOCs usingconfined LCs. (a) A LC droplet is encapsulated in a micro/nano structureformed in a polymeric material that absorbs VOCs. Upon exposure to VOCs,the polymer structure changes to an anisotropic shape due to theconstrained boundary conditions. (b) As a result, the LC undergoeschange in orientation inside the modified structure.

FIG. 35 is a schematic showing a basic principle of detection of VOCsusing dewetting-induced orientational transition of LCs. A glasssubstrate is coated with polymer (PS) and a film of LC is supported onthe polymer film When the film is exposed to VOCs, the film dewets thesurface and the orientation of LC changes.

FIG. 36 is a schematic showing a basic principle of detection of VOCsbased on structural changes on the surface supporting a LC film. Apolymer film that is known to absorb target analyte is deposited andmechanically sheared to generate micro-structures on the surface. Thesemicrostructures align LC in a pre-defined direction. Absorption of VOCsinto the polymer film erases the microstructures inducing a change inorientation of LCs film.

FIG. 37A-C is a schematic showing a basic principle of detection of VOCsusing orientation transition induced by dissolution of ionic salt intoLC film A thin film of ionic salt is deposited on self-assembledmonolayer formed on a gold coated surface. When the LC film is exposedto target VOC, the ionic salt radially dissolves into the LC filmthereby inducing orientational transition in the LC film

FIG. 38 is a schematic showing a basic principle of detection of VOCsusing orientation transition induced by localized concentration atdefect sites in LC. A thin film of LC is supported on a rough surfacethat generates local defects in the LC at the microscopic level, yetproviding uniform alignment on macroscopic level. When the surface isexposed to VOCs, the microscopic defects locally concentrate the VOCmolecules at the defect sites leading to ordering transition in the LCfilm

FIG. 39 is a schematic showing a basic principle of detection of VOCsutilizing swelling of polymer beads suspended in LC. Polymer beads aresuspended in a LC film that is supported on a surface providing uniformalignment of LC. Absorption of VOC in the polymer beads swells the beadsand induces distortion in the LC alignment around the beads. Thedistorted LC around these beads scatters light.

FIG. 40 is a schematic showing an experimental system for exposingsensors to VOC.

FIG. 41 is a plot showing the response of a LC sensor exposed to 5000ppm toluene. The light intensity transmitted through the sensor betweencrossed polarizers (mean gray scale intensity-MGSI) was measured as afunction of exposure time. The inset shows the visual appearance of thesensor between crossed polarizers before (bright) and after (dark) thevapor exposure. The sensor reverted back to initial appearanceimmediately after the toluene supply was ceased.

FIG. 42A-D shows optical images of LC cells prepared with polystyrene(PS) and low molecular weight PS (low MW PS) polymer coated on a glasssurface by pairing withTridecafluoro-1,1,2,2-tetrahydroocty9-trichlorosilane (OTS) coatedsubstrates. The polymer coated substrates were un-rubbed (a and c) andrubbed (b and d). These images were taken with the LC cells betweencrossed polarizers (A: analyzer and P: polarizer) with the rubbingdirection oriented at 0° (left) and 45° (right) with respect to thecrossed polarizers

FIG. 43A shows optical images of LC sandwich cells prepared with rubbedpolystyrene (PS) and OTS coated substrates “before” (left) and “after”(right) 20 hours of toluene exposure. The sandwich cells were exposed to8605 ppm (top), 5020 ppm (middle), and 0 ppm (bottom) toluene,respectively. Optical images were taken with the rubbing direction at 0°(left) and 45° (right) with respect to the crossed polarizers.

FIG. 43B shows the raw images of LC sandwich cells that were collectedat different time intervals while exposed to 8605 ppm (top), 5020 ppm(middle), and 0 ppm (bottom) toluene, respectively.

FIG. 43C shows the light intensity transmitted through the LC sandwichcells between crossed polarizers (mean gray scale intensity-MGSI)measured as a function of exposure time.

FIG. 44A-D shows optical images of LC cells prepared withpoly(vinyl)acetate (PVAc) and OTS coated substrates. The PVAc coatedsubstrates were (a) un-rubbed & unexposed, (b) un-rubbed & exposed, (c)rubbed & unexposed, and (d) rubbed & exposed. 28,680 ppm toluene wasused for exposure prior to cell fabrication. Optical images were takenwith rubbing direction at 0° and 45° with respect to crossed polarizers.

FIG. 45A-E shows optical images of LC cells prepared with rubbed PVAcand OTS coated substrates. The PVAc coated substrates were rubbed fivetimes or only once and exposed to toluene prior to cell fabrication. (a)rubbed (5 times) & exposed (8605 ppm), (b) rubbed (1 time) & exposed (0ppm), (c) rubbed (1 time) & exposed (8605 ppm), (d) rubbed (1 time) &exposed (5020 ppm), and (e) rubbed (1 time) & exposed (2868 ppm).Optical images were taken with rubbing direction at 0° and 45° withrespect to crossed polarizers.

FIG. 46A-D shows images of LC(E7) alignment of the exposed and unexposedoptical cells prepared using rubbed or unrubbed SC-F103 polymer(Seacoast Science Inc.) coated substrates. For rubbed polymer surfacesrubbing was done with a piece of velvet cloth rubbed one way for fivetimes before LC addition. A) Images of unexposed sandwich cells madewith different types of SC-F103 surfaces on glass, a) uncoated; b)unrubbed SC-F103; c) rubbed SC-F103; d) images of the rubbed SC-F103cell taken with rubbing direction at 0° and 45° with respect to crossedpolarizers. B) Images of a cell prepared with unrubbed SC-F103 coatedsurface before and after exposure to 5020 ppm toluene for 26 hours.Similar images were collected using the cells made with rubbed SC-F103coated surface before and after 23 hours of exposure to C) 5020 ppmtoluene and D) dry nitrogen, respectively.

FIG. 47 shows long chain hydrocarbons tested in experiments describedherein. FIG. 48A-B shows in (A) a schematic for making a thinhydrocarbon film on a 3″×1″ glass slide and in (B) the images of 1″×1″glass slides with LC (E7) coated on top of a hydrocarbon (C18) layerbetween crossed polarizers.

FIG. 49A-B shows (A) images of an LC sensor collected while exposed to5020 ppm toluene vapor. (B) Images of the same sensor collected off linebefore and after 19 hours of toluene exposure.

FIG. 50A-B shows images of A) a portion of the sandwich cells containing(i) octadecane and E7 and (ii) octadecane alone from 5 hours of 5020 ppmtoluene exposure. B) 5020 ppm toluene exposure to the E7 andE7-octadecane mixture spotted on a glass piece.

FIG. 51 shows images from experiments testing 5020 ppm toluene exposureto a sandwich cell prepared by pairing a substrate coated with C24 filmand an OTS coated glass pieces. The gap between two substrates wasfilled with LC E7.

FIG. 52A-D shows the detection of toluene vapor using PDLCs. (a) to (c)Microscopic pictures of PDLCs (a) before, (b) during, and (c) afterexposure to a high concentration of toluene. (d) The optical response ofPDLC sensors to 8600 ppm toluene in comparison to pure E7 sensors.

FIG. 53A-C shows images showing the morphology of PDLC droplets (a)before incubation at 50° C. for 2 minutes (b) after incubation at 50° C.for 2 minutes and after exposure to 8600 ppm toluene. The images weretaken with a polarizing microscope with 50× magnification objective.

FIG. 54 is a plot showing optical response from PDLC sensors atdifferent concentrations of toluene diluted in dry nitrogen. One sensorwas exposed to nitrogen for long time. One sensor used exposed to 4000PPM toluene was exposed to 8000 PPM toluene after 5 minutes. Similarly,one sensor exposed to 8000 PPM toluene was exposed to dry nitrogen after5 minutes.

FIG. 55 is a plot showing optical response from PDLC sensor uponsequential exposure to 4000 ppm toluene and dry N₂.

FIG. 56 is a schematic and series of images showing change in the PDLCdroplets supported on rubbed polymer film. The images show theappearance of PDLC droplets formed on rubbed surfaces before and afterexposure to toluene at different concentrations. All these exposureexperiments were performed at 45% relative humidity.

FIG. 57A-D is a series of images showing the macroscopic appearance ofPDLC droplets (a) before exposure, (b) after exposure to 8000 ppmtoluene for 8 minutes, (c) same sensor chip rotated by 45°, and (d)rotated chip exposed to 8000 ppm toluene after 8 minutes. All theseexposure experiments were performed at 45% relative humidity.

FIG. 58 is a plot showing the optical response from PDLC droplets formedon rubbed surface to toluene at different concentrations.

FIG. 59 is a series of images showing the alignment of LC on untreatedglass surfaces and surfaces coated with PS paired with different topsurfaces. The images were taken with polarizing optical microscopic with4× and 50× magnifications (as shown) and a camera before and afterannealing the LC cells for 10 minutes inside an oven at 70° C. Theinsets on the second column show conoscopic images.

FIG. 60 comprises images showing the microscopic (4×) change inappearance of a LC cell upon thermal treatment. The formation of themicroscopic domains resemble a typical pattern observed in dewetting ofpolymer films.

FIG. 61 provides images showing the appearance of a sensor upon exposureto saturated vapor of toluene.

FIG. 62 is a series of images showing the macroscopic appearance ofsensor upon exposure to 5000 ppm toluene (at 50% RH) for differentdurations. The last two images show the microscopic appearance afterovernight exposure to 5000 ppm toluene (50% RH) at 4× and 50×magnification, respectively.

FIG. 63 shows the macroscopic (camera) and microscopic appearance ofsensor fabricated on micropillared substrate. The polarizing opticalmicroscopic (POM) image on the left shows the bright lines due to planaralignment of LC.

FIG. 64A-E shows the appearance of micro-pillared sensor before andafter exposure to nitrogen (a: before exposure) and (b: after 30 minutesexposure) and 2000 ppm toluene (c: before exposure), (d: 30 minutesexposure) and (e: after 4 hour exposure). Plot (f) shows a quantitativemeasurement of the response.

It is to be understood that the figures are not necessarily drawn toscale, nor are the objects in the figures necessarily drawn to scale inrelationship to one another. The figures are depictions that areintended to bring clarity and understanding to various embodiments ofapparatuses, systems, and methods disclosed herein. Wherever possible,the same reference numbers will be used throughout the drawings to referto the same or like parts. Moreover, it should be appreciated that thedrawings are not intended to limit the scope of the present teachings inany way.

DETAILED DESCRIPTION

Provided herein is technology relating to detecting gaseous analytes andparticularly, but not exclusively, to devices and methods related todetecting gaseous analytes by monitoring changes in liquid crystals uponexposure to the gaseous analytes.

The section headings used herein are for organizational purposes onlyand are not to be construed as limiting the described subject matter inany way.

In this detailed description of the various embodiments, for purposes ofexplanation, numerous specific details are set forth to provide athorough understanding of the embodiments disclosed. One skilled in theart will appreciate, however, that these various embodiments may bepracticed with or without these specific details. In other instances,structures and devices are shown in block diagram form. Furthermore, oneskilled in the art can readily appreciate that the specific sequences inwhich methods are presented and performed are illustrative and it iscontemplated that the sequences can be varied and still remain withinthe spirit and scope of the various embodiments disclosed herein.

All literature and similar materials cited in this application,including but not limited to, patents, patent applications, articles,books, treatises, and internet web pages are expressly incorporated byreference in their entirety for any purpose. Unless defined otherwise,all technical and scientific terms used herein have the same meaning asis commonly understood by one of ordinary skill in the art to which thevarious embodiments described herein belongs. When definitions of termsin incorporated references appear to differ from the definitionsprovided in the present teachings, the definition provided in thepresent teachings shall control.

Definitions

To facilitate an understanding of the present technology, a number ofterms and phrases are defined below. Additional definitions are setforth throughout the detailed description.

Throughout the specification and claims, the following terms take themeanings explicitly associated herein, unless the context clearlydictates otherwise. The phrase “in one embodiment” as used herein doesnot necessarily refer to the same embodiment, though it may.Furthermore, the phrase “in another embodiment” as used herein does notnecessarily refer to a different embodiment, although it may. Thus, asdescribed below, various embodiments of the technology may be readilycombined, without departing from the scope or spirit of the technology.

In addition, as used herein, the term “or” is an inclusive “or” operatorand is equivalent to the term “and/or” unless the context clearlydictates otherwise. The term “based on” is not exclusive and allows forbeing based on additional factors not described, unless the contextclearly dictates otherwise. In addition, throughout the specification,the meaning of “a”, “an”, and “the” include plural references. Themeaning of “in” includes “in” and “on.”

As used herein, the term “wavefront” refers to a line of demarcationthat is observable between a region of ordered liquid crystal and aregion of disordered liquid crystal. In many cases, the wavefront isvisually detectable. However, the location of the wavefront can also bedetected by image analysis procedures.

As used herein, the term “ligand” refers to any molecules that bind toor can be bound by another molecule.

As used herein, the term “detection region” refers to a discreet areathat is designated for detection of an analyte in a sample.

As used herein, the terms “material” and “materials” refer to, in theirbroadest sense, any composition of matter.

As used herein, the term “field testing” refers to testing that occursoutside of a laboratory environment. Such testing can occur indoors oroutdoors at, for example, a worksite, a place of business, public orprivate land, or in a vehicle.

As used herein, the term “nanostructure” refers to a microscopicstructure, typically measured on a nanometer scale. Such structuresinclude various three-dimensional assemblies including, but not limitedto, liposomes; films; multilayers; braided, lamellar, helical, tubular,and fiber-like shapes; and combinations thereof. Such structures can, insome embodiments, exist as solvated polymers in aggregate forms such asrods and coils. Such structures can also be formed from inorganicmaterials, such as prepared by the physical deposition of a gold filmonto the surface of a solid, proteins immobilized on surfaces that havebeen mechanically rubbed, and polymeric materials that have been moldedor imprinted with topography by using a silicon template prepared byelectron beam lithography.

As used herein, the term “self-assembling monomers” refers to moleculesthat spontaneously associate to form molecular assemblies. In one sense,this can refer to surfactant molecules that associate to form surfactantmolecular assemblies. The term “self-assembling monomers” includessingle molecules and small molecular assemblies, whereby the individualsmall molecular assemblies can be further aggregated (e.g., assembledand polymerized) into larger molecular assemblies.

As used herein, the term “linker” or “spacer molecule” refers tomaterial that links one entity to another. In one sense, a molecule ormolecular group can be a linker that is covalent attached to two or moreother molecules (e.g., linking a ligand to a self-assembling monomer).

As used herein, the term “bond” refers to the linkage between atoms inmolecules and between ions and molecules in crystals. The term “singlebond” refers to a bond with two electrons occupying the bonding orbital.Single bonds between atoms in molecular notations are represented by asingle line drawn between two atoms (e.g., C—C). The term “double bond”refers to a bond that shares two electron pairs. Double bonds arestronger than single bonds and are more reactive. The term “triple bond”refers to the sharing of three electron pairs. As used herein, the term“ene-yne” refers to alternating double and triple bonds. As used hereinthe terms “amine bond,” “thiol bond,” and “aldehyde bond” refer to anybond formed between an amine group (e.g., a chemical group derived fromammonia by replacement of one or more of its hydrogen atoms byhydrocarbon groups), a thiol group (e.g., sulfur analogs of alcohols),and an aldehyde group (e.g., the chemical group —CHO joined directlyonto another carbon atom), respectively, and another atom or molecule.

As used herein, the term “covalent bond” refers to the linkage of twoatoms by the sharing of two electrons, one contributed by each of theatoms.

As used herein, the terms “optical anisotropy” and “birefringence” referto the optical property of having a refractive index that depends on thepolarization and propagation direction of light. Optically anisotropicmaterials are said to be birefringent.

The anisotropy in optical properties of liquid crystals gives rise tooptical birefringence, that is, different refractive indices whenmeasured with different polarization directions.

As used herein, the term “magnetic anisotropy” refers to havingdifferent magnetic properties for different directions of magneticfields. Magnetic anisotropy produces different magnetic susceptibilitiesin a material when measured with different magnetic field directions.

As used herein, the term “dielectric anisotropy” refers to havingdifferent dielectric properties for different directions of electricfields. Dielectric anisotropy produces different dielectric constants ina material when measured with different electric field directions.

As used herein, the term “spectrum” refers to the distribution ofelectromagnetic (e.g., light) energies arranged in order of wavelength.

As used the term “visible spectrum” refers to light radiation thatcontains wavelengths from approximately 360 nm to approximately 800 nm.

As used herein, the term “substrate” refers to a solid object or surfaceupon which another material is layered or attached. Solid supportsinclude, but are not limited to, glass, metals, gels, and filter paper,among others.

As used herein, the terms “array” and “patterned array” refer to anarrangement of elements (e.g., entities) onto or into a material ordevice. For example, depositing several types of liquid crystals intodiscrete regions on an analyte-detecting device would constitute anarray.

As used herein, the term “in situ” refers to processes, events, objects,or information that are present or take place within the context oftheir natural environment.

As used herein, the term “sample” is used in its broadest sense. In onesense it can refer to a biopolymeric material. In another sense, it ismeant to include a specimen or culture obtained from any source, as wellas biological and environmental samples. Biological samples may beobtained from animals (including humans) and encompass fluids, solids,tissues, and gases. Biological samples include blood products, such asplasma, serum and the like. Environmental samples include environmentalmaterial such as surface matter, soil, water, crystals, and industrialsamples. These examples are not to be construed as limiting the sampletypes applicable to the present technology.

As used herein, the term “liquid crystal” refers to a thermodynamicstable phase characterized by anisotropy of properties without theexistence of a three-dimensional crystal lattice, generally lying in thetemperature range between the solid and isotropic liquid phase.

As used herein, the term “mesogen” refers to compounds that form liquidcrystals, including rod-like or disc-like molecules that are componentsof liquid crystalline materials.

As used herein, “thermotropic liquid crystal” refers to liquid crystalsthat result from the melting of mesogenic solids due to an increase intemperature. Both pure substances and mixtures form thermotropic liquidcrystals.

“Lyotropic,” as used herein, refers to molecules that form phases withorientational and/or positional order in a solvent. Lyotropic liquidcrystals can be formed using amphiphilic molecules (e.g., sodiumlaurate, phosphatidylethanolamine, lecithin). The solvent can be water.

“Metallotropic,” as used herein, refers to metal complexes of organicligands that exhibit liquid crystalline character. Thermotropicmetallomesogens have been made that incorporate many metals. They can berodlike (calamitic) and disklike (discotic). The ligand can bemonodentate (e.g., 4-substituted pyridines), bidentate (e.g.,beta-diketonates, dithiolenes, carboxylates, cyclometalated aromaticamines), or polydentate (e.g., phthalocyanines, porphyrins). The ligandsinfluence the mesophase character based on molecular shape andintermolecular forces. The metallomesogens provide a rigid core, whichis typically unsaturated and either rod-or disklike in shape, andseveral long hydrocarbon tails where the metal atom is usually at ornear the center of gravity of the molecule. Metallotropic liquidcrystals, acting through the metal moiety, can be tuned to capturedifferent target analytes by different methods including but not limitedto displacement, redox reactions, and ligand formation.

As used herein, the term “heterogenous surface” refers to a surface thatorients liquid crystals in at least two separate planes or directions,such as across a gradient.

As used herein, “nematic” refers to liquid crystals in which the longaxes of the molecules remain substantially parallel, but the positionsof the centers of mass are randomly distributed. Nematic liquid crystalscan be substantially oriented by a nearby surface.

“Chiral nematic,” as used herein refers to liquid crystals in which themesogens are optically active. Instead of the director being heldlocally constant as is the case for nematics, the director rotates in ahelical fashion throughout the sample. Chiral nematic crystals show astrong optical activity that is much greater than can be explainedsolely on the bases of the rotatory power of the individual mesogens.When light equal in wavelength to the pitch of the director impinges onthe liquid crystal, the director acts like a diffraction grating,reflecting most and sometimes all light incident on it. If white lightis incident on such a material, only one color of light is reflected andit is circularly polarized. This phenomenon is known as selectivereflection and is responsible for the iridescent colors produced bychiral nematic crystals.

“Smectic,” as used herein, refers to liquid crystals that aredistinguished from “nematics” by the presence of a greater degree ofpositional order in addition to orientational order. In a smectic phasethe molecules spend more time in planes and layers than they do betweenthese planes and layers. “Polar smectic” layers occur when the mesogenshave permanent dipole moments. In the smectic A2 phase, for example,successive layers show anti ferroelectric order, with the direction ofthe permanent dipole alternating from layer to layer. If the moleculecontains a permanent dipole moment transverse to the long molecularaxis, then the chiral smectic phase is ferroelectric. A device utilizingthis phase can be intrinsically bistable.

“Frustrated phases,” as used herein, refers to another class of phasesformed by chiral molecules. These phases are not chiral; however, twistis introduced into the phase by an array of grain boundaries. A cubiclattice of defects (where the director is not defined) exists in acomplicated, orientationally ordered twisted structure. The distancebetween these defects is hundreds of nanometers, so these phases reflectlight just as crystals reflect X-rays.

“Discotic phases” are formed from molecules that are disc shaped ratherthan elongated. Usually these molecules have aromatic cores and sixlateral substituents. If the molecules are chiral or a chiral dopant isadded to a discotic liquid crystal, a chiral nematic discotic phase canform.

Embodiments of the Technology

The present technology relates to detecting gaseous compounds using aliquid crystal assay format and a device utilizing liquid crystals aspart of a reporting system. Liquid crystal-based assay systems anddevices (LC assays) are described, e.g., in U.S. Pat. No. 6,284,197;Int'l App. Pub. Nos. WO 2001/061357; WO 2001/061325; WO 1999/063329;Gupta et al. (1998) Science 279:2077-2080; Kim et al. (2000)“Orientations of Liquid Crystals on Mechanically Rubbed Films of BovineSerum Albumin: A Possible Substrate for Biomolecular Assays Based onLiquid Crystals” Analytical Chemistry 72: 4646-4653; Skaife et al.(2000) “Quantitative Interpretation of the Optical Textures of LiquidCrystals Caused by Specific Binding of Immunoglobulins to Surface-BoundAntigens” Langmuir 16: 3529-3536; Gupta et al. (1999) “Using Droplets ofNematic Liquid Crystal To Probe the Microscopic and Mesoscopic Structureof Organic Surfaces” Langmuir 15: 7213-7223; and Shah et al. (2001)“Principals for Measurement of Chemical Exposure Based onRecognition-Driven Anchoring Transitions in Liquid Crystals” Science293: 1296-99, all of which are incorporated herein by reference.

U.S. Pat. No. 6,284,197 and Shah et al, supra, describe the detection ofchemical molecules with a liquid crystal assay format that relies on anorientational change in the LC following the interaction of the chemicalmolecules with a functionalized surface on which the LC has beenoverlaid. Moreover, liquid crystal assays are also used for detectinggaseous compounds that interact directly or indirectly with the LCitself to produce a phase transition of the LC material. The use ofdifferent LCs that provide different functional moieties and/or reactivegroups, or the use of LC compositions comprising a dopant, in the assaysprovide materials that identify gaseous compounds through theinteraction of the gaseous compound with the functional moieties on theLC and/or the dopant. Furthermore, the liquid crystal assay devices ofthe present technology find use to quantify gaseous compounds.

In some embodiments, the detection of analytes or their derivatives ingas phase is accomplished through a direct interaction of the analytewith the LC. Depending upon the target analyte, some embodiments provideLCs that are synthesized to have a functional group that specificallyinteracts or reacts with the analyte. The liquid crystal can either besupported on a surface or in a small bulk amount through which theanalyte is passed. The present technology is not limited to thedetection of any particular analyte in gas phase. Indeed, the detectionof a variety of analytes is contemplated. Exemplary analytes are nitricoxide, formaldehyde, and hydrogen sulfide. A number of LCs withdifferent functional moieties is commercially available. Some of theseLCs have suitable reactive moieties that are selective for some targetanalytes. For example, MBBA (N-(4-methoxybenzylidene)-4-butylaniline andEBBA (N-(4-ethoxybenzylidene)-4-butylaniline) have functional groupssimilar to the aniline group that can be used for detectingnitrate-based gases. A number of azomethine-type LCs (see, e.g., Hiokiet al. (2004) Tetrahedron Letters 45: 7591-7594), polyaniline-basedpolymers (J. Phys. Chem. B 108: 8894-8899), and polyaniline-basedmoieties and polyimides (Journal of Polymer Science: Part A: PolymerChemistry 40: 1583-1593) have been synthesized. The interaction betweenthe analyte and the LC can be physical in nature or based on a chemicalreaction. The interaction of the target analyte with the LC can manifestas a change in a physical property of the LC (e.g., a change in thephase transition temperature, optical birefringence, dielectricanisotropy, magnetic anisotropy, or a change in the orientation of theLC on a surface) that can be detected using a variety of instrumentscapable of detecting these physical changes.

In some embodiments, the LC molecules are oriented on a chemicallyfunctionalized surface having a surface chemistry that is known tointeract with the target analytes. When the sensor surface is exposed toa test environment, the analyte diffuses through the LC film andinteracts with the surface chemistry. As a result, the orientation ofthe LC on the modified surface changes, thus leading to a change in theoptical properties of the LC film.

In some embodiments, the LC sensor comprises an LC film that issupported by a single chemically functionalized surface and the whole LCfilm is exposed to the test environment. Upon exposure, the analytemolecules diffuse through the LC film and bind to the surface chemistryand the LC molecules change orientation. As a result, the opticalproperties and appearance of the LC film change in real time. Dependingon the surface chemistry/analyte combination, the response can bereversible or irreversible. This embodiment allows for the sensitivedetection of analytes. In some embodiments, the dynamic response of thesensor is monitored by measuring the response time (e.g., the time ittakes for the sensor to respond). The response time is a function of theconcentration of the analyte and is used as a parameter to assess thequantitative response of the sensor.

Some embodiments utilize a thin film of LC supported between twochemically functionalized surfaces with openings from one or more sidesof the sensor. When the monitor is exposed to the test environment, theanalyte now will have to diffuse from the side of the sensor (as opposedto from the top of the LC film). Therefore, only the cross-section ofthe LC film is exposed to the test environment. As the analyte diffusesacross the film, it interacts with the surface chemistry, therebyinducing a change in the orientation of the LC. This change appears as abright front on the sides of the sensor open to the test environmentthat propagates inward into the LC film as the exposure time increases.Because of the macroscopic diffusion dimension involved, the response isirreversible and it provides a cumulative measure of the analyte. Ameasurable response is obtained after macroscopic lateral diffusion ofthe analytes through the LC film

In some embodiments, the technology is related to detecting VOCs usingmaterials and structures in conjunction with liquid crystals (LCs) thatfind use in simple, inexpensive sensors for unambiguous detection ofVOCs. In particular, changes in physical properties of a LC material incontact with these materials and structures, upon exposure to VOCs, aredetected visually, by simple light intensity measurement, or bymeasuring other measurable physical parameters (such as dielectricanisotropy, light scattering, etc.) associated with the state of LCmaterial.

I. Liquid Crystals

The technology is related to sensors comprising a liquid crystal (LC).The technology is not limited in the liquid crystal used and, further,the technology provides various embodiments in which any known or yetdiscovered LC is used according to the technology as it is describedherein. Any compound or mixture of compounds that forms a mesogeniclayer can be used in conjunction with the present technology. Themesogens can form thermotropic or lyotropic liquid crystals. Themesogenic layer can be either continuous or it can be patterned. In someembodiments, the LC comprises a compound comprising a Schiff base. Insome embodiments, the compound is a diazo compound, an azoxy compound, anitrone, a stilbene, a tolan, an ester, or a biphenyl. For example, insome embodiments, the LC comprises a compound according to thestructure:

wherein R and R′ are independently selected from alkyl, lower alkyl,substituted alkyl, aryl groups, acyl, halogens, hydroxy, cyano, amino,alkoxy, alkylamino, acylamino, thioamido, acyloxy, aryloxy,aryloxyalkyl, mercapto, thia, aza, oxo, saturated cyclic hydrocarbon,unsaturated cyclic hydrocarbons, heterocycle, arylalkyl, substitutedaryl, alkylhalo, acylamino, mercapto, substituted arylalkyl, heteroaryl,heteroarylalkyl, substituted heteroaryl, substituted heteroarylalkyl,substituted heterocyclic, and heterocyclicalkyl. In some embodiments,Xis selected from C1 to C10, —C═N—, —N═N—, —N═N(O)—, —C═N(O)═N(O)—,—CH═NO—, —HC═CH—, —C≡C—, and —OC(O)—.

In some embodiments the LC is a nematic LC (e.g., E7) and in someembodiments the LC is a smectic liquid crystal (e.g., 8CB). In someembodiments, the

LC is a thermotropic LC and in some embodiments the LC is a lyotropicLC. Additional examples of liquid crystals, include, but are not limitedto, 4-cyano-4′-pentylbiphenyl (5CB) and 7CB. A large listing of suitableliquid crystals is presented in “Handbook of Liquid Crystal Research” byPeter J. Collings and Jay S. Patel, Oxford University Press, 1997, ISBN0-19-508442-X, incorporated herein by reference.

The technology comprises use of polymeric liquid crystals in someembodiments. In some embodiments, the LC is a cholesteric liquid crystaland in some embodiments the LC is a ferroelectric liquid crystal. Insome embodiments, the LC is smectic C, smectic C*, a blue phase, and/ora smectic A LC. It is further envisioned that LCs useful in theinvention may further include additions of dopants such as, but notlimited to, chiral dopants as described by Shitara H, et al. (ChemistryLetters 3: 261-262 (1998)) and Pape, M., et al. (Molecular Crystals andLiquid Crystals 307: 155-173 (1997)). The introduction of a dopantpermits manipulation of the liquid crystal's characteristics including,but not limited to, the torque transmitted by the liquid crystal to anunderlying surface. Other dopants, such as salts, permit manipulation ofthe electrical double layers that form at the interfaces of the liquidcrystals and thus permit manipulation of the strength of anchoring ofthe liquid crystal at the interface. A number of methods for preparinginterfaces between liquid crystals and aqueous phases lie within thescope of the present invention. An approximately planar interface can beprepared by a film of liquid crystal in contact with an aqueous phase,or alternatively a curved interface can be prepared by using a dropletof liquid crystal dispersed in an aqueous phase. The scope of theinvention is not limited by the methods by which interfaces betweenaqueous phases and liquid crystals can be prepared by those skilled inthe art.

In some embodiments, the liquid crystals may preferably be selected fromMBBA, EBBA, E7, MLC-6812, MLC 12200, 5CB (4-n-pentyl-4′-cyanobiphenyl),8CB (4-cyano-4′octylbipheny9 and 4-(trans-4-heptylcyclohexyl)-aniline.

II Devices Devices Comprising a Defined Headspace (i.e., MicrofluidicCells)

In some embodiments, the devices according to the technology comprise aheadspace to control diffusion of analytes above the LC. In thisembodiment, the device comprises a substrate having a micropillared areathat is chemically functionalized with a surface chemistry that isspecific for the target analytes. The micropillared area is filled withthe LC using capillary action to form a thin (e.g., approximately 1 to20 microns, e.g., 5 microns) film This sensor substrate is then pairedwith a glass substrate with a headspace (e.g., having a height of 1 to100 microns, e.g., 20 microns) to allow controlled diffusion of thetargeted analytes above the LC film. In some embodiments, the headspaceis variable. In some embodiments, the first and second surfaces form acompartment having first and second open ends, wherein the headspace atthe first end is from 1 to 20 microns and the headspace at the secondend is from 21 to 100 microns. In some embodiments, the top surface isfunctionalized with(Tridecafluoro-1,1,2,2-tetrahydrooctyl)-trichlorosilane (OTS) on whichthe LC film does not spread. This minimizes or eliminates the formingand spreading of small LC droplets that could touch the top surface andobstruct the diffusion of gas across the head space. This embodiment ofthe device provides sensitive detection of analytes in real-time andprovides a cumulative detection of analytes. Additionally, in thisembodiment, varying the thickness of the headspace between the LC filmand the OTS coated surface provides control of the dynamic range of thedevice. In some embodiments, a spacer is placed between the two halvesof a sandwich type of a cell. In some embodiments, the spacer comprisesa material such as mylar or some polymer material having a definedthickness.

Devices Comprising a Channel

In some embodiments, devices according to the technology comprise achannel (e.g., a microfluidic channel) having a functionalized surface.In contrast to these embodiments of the technology, some conventional LCsensors are fabricated by supporting a thin film of LC on a chemicallyfunctionalized surface. When these sensors are exposed to theenvironment to be tested, the analyte diffuses through the LC film andthen interacts with the surface chemistry to change the LC orientation.However, in these configurations (e.g., with the LC film in place), theanalyte has to diffuse through the LC film to reach the surfacechemistry. In some cases, problems arise, especially with thesensitivity of the sensors. For example, since the analyte has todiffuse through the LC film, the LC acts as a diffusion barrier thatconsequently reduces the sensitivity of the device. As such, thesensitivity of detection is limited by the partition of the gas throughthe LC film Additionally, if the analyte reacts with the LC, someanalyte is consumed before it reaches the active surface.

Accordingly, provided herein are embodiments that address sensorsensitivity using a two-step process in which the chemicallyfunctionalized surface is first exposed to the analyte and then the LCis added to contact the modified surface. The region that had beenexposed to the analyte exhibits a different LC orientation relative tothe unexposed regions. This approach is, in particular, very effectiveif the analyte irreversibly reacts with the surface chemistry.

In some embodiments, the technology is related to a microfluidic device.Embodiments of the device according to the technology comprise welldefined microchannels formed on a PDMS (polydimethylsiloxane) slab thatis paired with a chemically functionalized surface. The gas samplecontaining the analyte is flowed through the microfluidic channeldefined between the PDMS slab and the chemically functionalized surface.After a predefined time, the PDMS channels are removed and a thin filmof LC is overlaid between an OTS coated slide and the now-reactedchemically functionalized surface. The length of the channel thatreacted with the analyte shows a different LC orientation (and thusappears different when interrogated by, e.g., polarized light) comparedwith the background or with an unreacted region of a channel. For afixed flow rate and exposure time, the length of the channel having achanged LC orientation is a function of concentration of the analyte.Thus, by measuring the length of the channel having a different LCorientation the concentration of the analyte is determined. Thisapproach not only allows a sensitive detection of analytes, but alsoprovides a quantitative method to determine an unknown concentration ofan analyte. Moreover, this approach provides efficient sampling ofanalytes by increasing the number of analyte molecules that come incontact with the chemically functionalized surface, therebysignificantly improving the sensitivity of detection of gases.

Dosimeters

In some embodiments, the sensors find use in a dosimeter for, e.g.,personal monitoring of a person to an analyte such as a toxic gas (e.g.,formaldehyde (HCHO), H₂S, NO₂, organic compounds, etc.). For example, itis contemplated that some embodiments of the technology allow detectionof a concentration range of an analyte when exposed for a particularamount of time, e.g., 0.15 to 10 ppm HCHO after an 8-hour exposure.Devices are constructed and verified by exposing the dosimeters toanalyte (e.g., HCHO gas) inside an exposure chamber by delivering theanalyte at a known concentration and flow rate (e.g., HCHO at a nominalflow rate of 200 ml/minute) to minimize the linear velocity (<1cm/minute) at the dosimeter surface and thus mimic a static exposure.After exposure, optical images of the dosimeter are captured using adigital camera and analyzed using image processing software to measurethe response, e.g., by the decrease in light intensity (brightness)between the crossed polarizers or by the increase in the width of thedark front.

While an understanding of the mechanism is not required to practice thetechnology, in some embodiments the sensitivity of detection depends,for example, on the chemical functionality of the LC composition and/orthe thickness of the LC film For example, data collected during thedevelopment of particular embodiments of the technology demonstrated aselectivity of MBBA, a LC known to significant concentrations of4-methoxybenzaldehyde and 4-butylaniline, for selective detection ofHCHO. The specificity of the response of the MBBA-based LC to HCHO isconsistent with the response being caused by a reaction between the4-butylaniline present in MBBA. This reaction, driven by mass action inthe presence of HCHO results in a Schiff's base compound that causes LCto undergo phase transition and the LC film appears dark when viewedbetween crossed polars. Accordingly, it is contemplated that other LCshaving different compositions and chemical functionalities, when mixedwith different amines provide sensitive detection of HCHO (Table 1).

TABLE 1 Properties of LCs and amines Liquid Functional T_(NI) CrystalsSource Composition Group (° C.) MBBA Aldrich schiff's base imine, 45ether 5CB Merck cyano biphenyl nitrile 35 E7 Merck cyano bi & terphenylsnitrile 65 TL 205 Merck fluorinated bi & terphenyls fluorine 87 Amines(Aldrich)

Exemplary properties of some mesogens are provided in Table 2 (phasetransition temperatures TN are provided in degrees Celsius).

Moreover, it is contemplated that the response to analyte varies withthe thickness of the LC film. Accordingly, in some embodiments thedevices comprise a micropillared substrate (as described herein) thatdetermines the thickness of the LC film. For example, some embodimentscomprise micro-pillars having heights of 2, 5, or 10 microns as producedusing conventional photolithography. As described for the technologyprovided, substrates are filled with an LCs (e.g., as provided in inTables 1 or 2) using capillary action and the LC-filled substrates arepaired with a clean glass substrate with a fixed head-space (e.g., 25 or45 microns) to form a dosimeter badge.

Various combinations of LC composition and film thickness arecontemplated to provide various sensitivities to a number of analytes.In particular, different functional groups (e.g., for those embodimentsthat comprise a functional group) provide for the specific detection ofdifferent analytes. The thickness of the LC is related to the rate ofinteraction of the analyte with the functional groups and the size ofthe headspace is related to the rate of exposure of the device to theanalyte.

The evolution of a dark front results in a decrease in the measuredbrightness. In some embodiments a change of at least 10% from an initialunexposed value is a response to analyte exposure. For example, asapplied to the detection of HCHO, preferred embodiments detect 0.15 ppmof HCHO after an 8-hour exposure. Fabricating embodiments comprisingvarious combinations of LC, film thickness, and a defined headspaceprovide for control of the dynamic range of the devices. For example,some embodiments provide a dynamic range of 0.15 to 10 ppm (e.g., as forsome embodiments that detect HCHO) and some embodiments provide adynamic range of 0.1 to 15 ppm (e.g., as for some embodiments thatdetect H₂S). In some embodiments, the device (e.g., a dosimeter) isexposed to sample on 1, 2, 3, 4, or more edges while the remaining edgesare sealed to prevent or minimize the entrance of analyte through thoseedges. The total surface area of the LC exposed to a sample possiblycomprising an analyte is related to the response of the device to theanalyte. In some embodiments, the distance the dark front travels in tothe dosimeter is related to the response of the device to the analyte.

By testing embodiments of the devices using known amounts of analyte, adose-response curve is produced that correlate a measured response to ananalyte concentration. After measuring the device response to an unknownconcentration of analyte, the dose-response curve provides for thecalculation of the unknown concentration from the measured response.Statistical analysis of the response to known concentrations of analyteprovides an estimate of the error in a measurement of an unknown.

Testing the device response to potentially interfering compoundsverifies the specificity of the response. For example, devices specificfor a particular analyte do not respond or have a minimal response toother substances that are not the analyte. In some embodiments, an LCand/or functional group is chosen that shows a maximal response to thetarget analyte and a minimal or no response to non-target substances.Some embodiments comprise a filter (e.g., a membrane, adsorbent,zeolite, etc.) to prevent or minimize the response of the device toparticular non-analyte substances. In addition, dosimeters are tested toverify that temperature changes and gas flow rate do not adverselyaffect the device performance. For example, some embodiments provide ahousing for the device that controls the flow rate of the sampledenvironment to the sensing surface. The shape and configuration as wellas the size of the aperture that allows the passage of sample gas toaccess the LC device are related to the control of the flow rate.

Some embodiments provide a device that is responsive to severalsubstances, e.g., a class of compounds (e.g., organic, aliphatic,aromatic, halogen, etc.) or particular functional groups.

III Dectection Detection Based on Phase Transition

LC materials typically comprise rod-shaped organic molecules. Thesemolecules form anisotropic condensed phases that possess long-rangeorientational ordering (crystal-like) but lack positional ordering(liquid-like). The long-range ordering of molecules within an LC givesrise to anisotropic optical properties—so-called optical birefringence.Absorption of analytes (e.g., VOCs) into the LC phase disrupts thelong-range order of the LC, thus giving rise to a phase transition to anisotropic material. This phase transition, in turn, leads to distinctchanges in the optical appearance of the LC. FIG. 33 illustrates theprinciples underlying the LC sensor for detection of analytes using aphase transition. The sensor comprises a micrometer-thick film of LCsupported on a solid substrate decorated with polymeric micro-pillars (5μm diameter, 10 μm center-to-center spacing). The micropillars are usedto form mechanically robust thin films of the LC. Prior to exposure totoluene, the LC possesses a bright visual appearance between crossedpolarizers. When the sensor is exposed to the test environmentcontaining analytes, analyte molecules rapidly diffuse into the LC film(FIG. 33a ) to induce a nematic-to-isotropic phase transition. Theprocess of diffusion into the LC is rapid, predictable, and the phasetransition in LC is denoted by a striking change in optical appearanceof the sensor (FIG. 33b ). In other embodiments of the invention, theVOC causes a change from one liquid crystalline phase into anotherliquid crystalline phase. It is not necessary that the phase transitioninvolve an isotropic phase.

Detection Based on Structural Change of LC Confined in Micro/NanoStructures

When a LC droplet is confined by a micro and/or a nano structure, the LCmolecules inside the droplet assume a well-defined configuration that isdetermined by the properties of the material forming the structure, thedimensions of the structure, and the LC materials. The absorption ofanalyte molecules induces a change in the structure and dimensions ofthe confining structure. The structural changes in the confiningmaterial, in turn, induce a change in the ordering of the LC inside thecavity. By appropriate selection of the materials for confinement,structures for encapsulating LCs, and LC materials, the selectivity andsensitivity of detection is tuned. FIG. 34 shows a basic principlebehind this approach of detection where a LC droplet is confined insidea polymer matrix that is known to adsorb target analytes. In thisexample, the polymer material provides LC alignment perpendicular to theLC-polymer interface. The polymer matrix is confined between two rigidstructures so that it can deform only along the one direction (indicatedby the arrow). Upon exposure to analyte, the polymer deforms to ananisotropic shape inducing an ordering transition in the LC droplet.Various embodiments of the same basic principle are envisioned. Forexample, one embodiment involves fabrication of well-defined polymerdispersed LC structures that deform/change upon exposure to an analytesuch as a VOC.

Detection Based on Dewetting of Film Supporting LCs

The stability of a thin film of material deposited on a solid substratedepends on a number of parameters such as the surface energy of thesubstrate, the physical and chemical structure of the depositedmaterial, the thickness of the film, etc. Some polymeric materials suchas polystyrene form a stable film on glass or silicon. The PS film hasbeen shown to dewet if the thickness of the film is lower than acritical value or if the temperature of the film is raised above acritical value. Since the effect of exposing a polymer film to analytes(e.g., a VOC such as toluene) is very similar to heating (e.g., exposureto toluene lowers the glass transition of polymer), it is anticipatedthat exposure to some analytes will induce the dewetting of the film. Ifthe underlying substrate and the polymer materials are selected so thatthe orientation of LC on the substrate relative to the orientation of LCon the substrate coated with polymer is different, exposure to analytewill lead to a change in the orientation of the LC. The sensitivity ofdetection can be enhanced by selecting polymer material that has a highabsorption of analytes of interest. The basic principle behind thedewetting-induced orientational transition is schematically shown inFIG. 35. The main difference between the analyte-induced orientationaltransition and the dewetting approach is that in an analyte-inducedorientational transition the LCs remain in contact with the chemicallyfunctionalized surface before and after exposure. In this approach, thechemically functionalized surface (in this case polymer) is modified andphysically dewets the substrate. As a result, the LC comes in contactwith the underlying surface. The film that undergoes the dewettingtransition can be formed from a number of organic or inorganicmaterials.

Detection Based on Changes on Microscopic Structures on a Polymer FilmSupporting LC Film

The orientation of an LC at the LC-substrate interface is extremelysensitive to changes in physical and chemical properties of theLC-substrate interface. Rubbed polymer (such as polyimide) films havebeen widely used in LC display industries to achieve uniform planaralignment of LCs. Although the exact mechanism for the rubbing-inducedLC alignment is not fully understood, it is believed that anisotropicphysical interactions related to anisotropy in surface morphology areresponsible for uniform alignment of LCs on rubbed polymer surfaces.However, an understanding of the mechanism is not required to practicethe technology. Polymer materials, that are known to cause swelling as aresult of VOC absorption, can be deposited on a solid substrate andmechanically sheared to generate micro-structures similar to those usedfor LCD displays. These surfaces initially promote uniform alignment ofLCs. Absorption of analytes such as, e.g., VOCs, induces structuralchanges at the LC-surface interface. This structural change leads to achange in the orientation of the LC film. This principle was used (seeFIG. 36) to achieve a uniform alignment of LC on surfaces coated withpolymer films, e.g., films formed from poly(vinyl acetate) (PVAc) andpolystyrene (PS). It is contemplated that a polymeric or liquidcrystalline polymeric film can be stabilized (e.g., thermally,electrically, or mechanically) in a thermodynamic non-equilibrium state.In such a state, the film, upon exposure to analyte, relaxes to a lowerenergy state by releasing the stored energy. As a result, the LC filmsupported on this film undergoes orientational transition.

Detection Based on Dissolution of Ionic Compounds

Controlling the presence of ionic compounds in an LC provides afunctionality to detect analytes. Two approaches are contemplated.First, in some embodiments, the presence of certain ionic compounds inthe LC induces a LC orientation change at the LC-surface interface. Forexample, a surface prepared with tetrabutylammonium perchlorate wasshown to induce a homeotropic alignment as it dissolved into the LClayer. As the molecular weight and chain lengths of the quaternaryammonium compounds increases, their solubility in non-polar solvents(e.g., toluene) increases. An LC film supported on an appropriatequaternary ammonium salt coated surface is thus anticipated to dissolvethis salt upon toluene exposure and produce a change in the LCorientation. Second, in some embodiments, a cyanobiphenyl LCs (e.g.,SCB, E7) aligns perpendicularly on surfaces decorated with bivalent andtrivalent metal perchlorate salts. Besides perchlorate salts, only a fewother metal salts, such as tetraphenylborate (a metal salt compresing abulky anion), align LC perpendicular to the surface. In addition, thesesalts with large anions possess high solubility in toluene or othernon-polar organic solvents. In contrast to the metal perchlorate salts,bivalent and trivalent metals with anions such as acetate, chloride,nitrate, etc. are insoluble in non-polar solvents and are known to alignLCs parallel to the surface. Thus, a surface can be prepared from amixture of metal tetraphenylborate and a metal acetate salt in anadequate ratio to induce a homeotropic alignment initially. As thesurface is exposed to analytes such as a VOC the tetraphenylborate isanticipated to enter into the LC layer followed by a LC orientationchange due to a change in the surface composition. A schematic of theprinciple is shown in FIG. 37.

Detection Based on Orientational Transition Induced by LocalizedConcentration at Defect Sites in LC

In some embodiments, a thin film of LC is supported on a locally roughsurface (for example etched silicon dioxide) that possesses sharpdefects while promoting well defined orientation (for example, anorientation that is substantially perpendicular to the surface) of theLC. When this film of LC is exposed to an environment containing ananalyte (e.g., VOCs), the analyte molecules diffuse through the film ofLC and concentrate locally at the defect sites. When the localconcentration of analyte at the defect site is greater than thethreshold needed to induce an orientational transition, the LC filmundergoes the orientational transition (FIG. 38). Related embodimentsinvolve the use of a blue phase of a LC that is known have defect statesin the film itself (as opposed to the defect being in the surfacesupporting the film). These defect states concentrate the analytemolecules locally to induce melting at these defect sites.

Detection Based on Swelling of Polymer Beads Dispersed in LC

In some embodiments, micro and/or nano scale polymer beads composed ofmaterials that are known to swell by absorbing certain analytes (e.g.,VOCs) are suspended in the LC matrix. The dimension of the beads issmall enough not to distort the LC director when they are suspended. TheLC is uniformly aligned and the LC film does not scatter light. Whenthis film is exposed to an environment containing analyte, the beadsswell significantly due to absorption of analyte molecules. Once adimension of the beads exceeds a critical value the beads distort thedirector distribution in the LC matrix. As a result of distortion in thedirector configuration, the LC film scatters light (FIG. 39). Anextension of this approach is to use beads that are randomly distributedin the LC matrix. When the LC film with beads is exposed to an analyte,the density of the beads decreases (they become lighter) and theysubsequently float on the top of the LC surface. If the beads are chosento be opaque they will block the light transmitted through the LC film.

Detection of Gas Phase Compounds

The present technology provides methods and devices for the detection ofgas phase compounds in a sample. The only limitations on size and shapeare those that arise from the situation in which the device is used orthe purpose for which it is intended. In some embodiments, the devicescomprise a single substrate that is open to the environment on onesurface. The device can be planar or non-planar. The device can becylindrical in shape and in a linear or coiled format, and with one ortwo ends of the device open to the environment. Furthermore, it iswithin the scope of the present technology to use any number ofpolarizers, lenses, filters, lights, and the like to practice thepresent technology.

In some embodiments, devices comprise a mesogen. The present technologyis not limited to any particular mechanism of action. Indeed, anunderstanding of the mechanism of action is not necessary to practicethe present technology. Nevertheless, it is contemplated that the insome embodiments the mesogens forming the liquid crystal of the devicesof the present technology have an affinity for the targeted compound.This affinity causes a phase transition of the liquid crystal in thepresence of the target. Particular mesogens will transition from ahigher order to a lower order following interaction with differentmolecules. The devices of the present technology are designed so thatwhen gas phase compounds are present in a sample, the gas can enterdetection regions of the device where mesogens are arrayed and cause achange in the phase order of the mesogen by interacting with themesogen. This phase transition may be from one phase selected from thegroup consisting of an isotropic phase, a nematic phase, and a smecticphase to another phase selected from the group consisting of anisotropic phase, a nematic phase, and a smectic phase. The phasetransition induces a change in the liquid crystal (e.g., a nematicregion as opposed to an isotropic region) that can be detected in avariety of ways.

In some embodiments, the present technology provides substrates overlaidwith mesogens into which the gas phase compound diffuses leading to aphase transition of the mesogen. In other embodiments, the gas phasecompound interacts directly with a reactive moiety of the mesogen toinduce the phase change of the mesogen. In still other embodiments, thegas phase compound diffuses into a mesogen composition containing adopant and interacts with a reactive moiety on the dopant leading to aphase transition of the mesogen. In some embodiments, the gas phasecompound interacts with a surface (e.g., a functional group attached tothe surface) supporting the mesogen such that the interaction of the gasphase compound with the surface produces a change in orientation of themesogen.

Accordingly, in some embodiments, the present technology providessubstrates comprising at least one detection region comprising a mesogencomposition comprising a reactive moiety that binds to or otherwiseinteracts with a gas phase compound. In some embodiments, the detectionregions are discrete and created by arraying at least one reactivemoiety on the surface of the substrate. In some embodiments, a pluralityof mesogens with various reactive moieties is arrayed on the surface ofthe substrate so that multiplexed assays for a variety of gas phasecompounds can be conducted or so that different interactions with avariety of reactive moieties can be used as a signature for a particulargas phase compound. In some embodiments, a liquid handler is used todeposit the mesogen composition in the detection region.

In some embodiments, a second substrate is provided that is configuredopposite the first substrate so that a cell is formed. In someembodiments, the second substrate is also arrayed with a mesogencomposition comprising a reactive moiety, while in other embodiments,the second substrate is free of reactive moieties. In some embodiments,the mesogen compositions comprising a reactive moiety are arrayed on thefirst and second substrates so that when the first and second substratesare placed opposite each other the arrays match to form discretedetection regions.

In some embodiments, the cell that is formed by the first and secondsubstrates includes a space between the first and second substrates. Insome embodiments, the space is formed by placing a spacer (e.g., in someembodiments made of mylar) between the first and second substrates. Insome embodiments, the space is then filled with the desired liquidcrystal. In still other embodiments, the substrates are arranged so thata sample can interact with or enter into the detection regions. In someembodiments, the substrates are fixed (e.g., permanently or removably)to one another. The present technology is not limited to any particularmode of fixation. Indeed, a variety of modes of fixation arecontemplated. In some embodiments, the substrates are fixed to oneanother via adhesive tape. In preferred embodiments, the adhesive tapeis 8141 pressure sensitive adhesive (3M, Minneapolis, Minn.). In otherembodiments, the substrates are fixed to one another via a UV curableadhesive. In some preferred embodiments, the UV curable adhesive isPHOTOLEC® A704 or A720 (Sekisui, Hong Kong). In some embodiments, glassspacer rods are utilized with the UV curable adhesive to provide spacingbetween the two substrates. In some embodiments, the glass spacer rodsrange from about 5 μM to about 100 μM, preferably about 25 μM, inthickness. It has been found that UV-curable adhesives are preferable asin some instances the adhesive tape reacts with the liquid crystal.

In further embodiments, the substrates are arranged in a housing. Thehousing can comprise any suitable material, and is preferably made ofpolymeric material, for example, a plastic. In preferred embodiments,the housing is sealed to the environment except for an opening adjacentto the detection region or regions. The opening preferably allowsdiffusion of air to the detection region. In some embodiments, theopening allows introduction of a liquid sample wherein gas emitted froma constituent in the sample impinges on the substrates and can beinterrogated. In some embodiments, the opening is covered with a filtermaterial that allows diffusion of air to the detection region, but doesnot allow entry of particulate matter such as dust, dirt, liquid, andinsects into the detection region. In some embodiments, the filter is anaerosol filter that substantially prevents the introduction of aerosolsinto the detection region, but allows an analyte in vapor form to enterthe detection region. In still more preferred embodiments, the devicescomprise two or more filters positioned so as to allow air exchangethough the device, and in particular, through the detection region. Forexample, the filters can be arranged at either end of the detectionregion. In further embodiments, the housing is moveable between anexposure mode and a reading mode. In the exposure mode, the detectionregions are exposed to the environment, while in the reading mode,exposure to the environment is substantially or completely eliminated.It is envisioned that after the device has been exposed to theenvironment, the housing can be moved to the reading mode to preventfurther exposure to the environment prior to readout.

In still further embodiments, the devices of the present technologycomprise a unique identifier. In some embodiments, the unique identifieris a bar code. In other embodiments, the unique identifier is an RFIDchip. It is contemplated that the unique identifier can provideinformation such as a serial number, user identification, sourceidentification, and the like.

In use, the device is placed in an area where the gas phase compoundsare suspected of being present. The device is allowed to remain in placefor a period of time (the exposure period, e.g., from one or moreminutes to one or more hours to one or more days to one or more weeks ormore).

In other uses, a liquid sample that is biological or pharmaceutical innature and suspected of containing bacteria is introduced into thedevice. The sample is allowed to incubate for a period of time (e.g.,for the exposure period, e.g., from 15 minutes to 4 days). In apreferred use, the device receives a liquid sample and is incubated at37° C. for 1 hour with shaking to permit replication of bacteria thatleads to release of metabolic gases.

Following the exposure period, the cell is assayed for whether a changein the liquid crystal phase has occurred over one or more of thedetection regions. Although many changes in the mesogenic layer can bedetected by visual observation under ambient light, any means fordetecting the change in the mesogenic layer can be incorporated into, orused in conjunction with, the device. Thus, it is within the scope ofthe present technology to use lights, microscopes, spectrometry,electrical techniques and the like to aid in the detection of a changein the mesogenic layer. In some embodiments, the presence of gas phasecompounds is detected by a change in the color and texture of the liquidcrystal.

Accordingly, in those embodiments utilizing light in the visible regionof the spectrum, the light can be used to simply illuminate details ofthe mesogenic layer. Alternatively, the light can be passed through themesogenic layer and the amount of light transmitted, absorbed, orreflected can be measured. The device can utilize a backlighting devicesuch as that described in U.S. Pat. No. 5,739,879, incorporated hereinby reference. Light in the ultraviolet and infrared regions is also ofuse in the present technology. In other embodiments, the device, and inparticular the detection region, is illuminated with a monochromaticlight source (e.g., 660-nm LEDs). In some embodiments, the cell isplaced in between cross-polarized lenses and light is passed though thelenses and the cell. In still other embodiments, the detection region ismasked off from the rest of the device by a template or mask that isplaced over the device.

The devices of the present technology are useful for measuringcumulative exposure to gas phase compounds. In some embodiments,cumulative exposure is assayed by determining the advancement of awavefront in the detection region. It is contemplated that the wavefrontadvances from an opening associated with the detection region. Thedistance of advancement correlates to the degree of exposure to gasphase compounds and is thus quantitative. In particular, it iscontemplated that the rate of progress of the wavefront into thedetection region depends on the concentration of gas phase compound towhich the device is exposed. In some embodiments, the front movement inmillimeters is plotted against elapsed time in hours. The resulting plotobeys a linear fit (preferably with a coefficient of correlation ofgreater than 0.95) that is characteristic of the concentration of a gasphase compound in the sample (e.g., local atmosphere). In someembodiments, wavefront advancement is measured by capturing a digitalimage or video in real time of the detection region and determining thearea and length (e.g., in pixels) of the wavefront relative to theopening. In some preferred embodiments, the image is analyzed with animage manipulation and analysis program such as ImageJ (NIH). The pixelscan then be converted into a distance in millimeters if necessary. Inother embodiments, the image is analyzed by converting the image with a%white command so that the area in which the liquid crystal has beendisrupted by the gas phase compound appears white. The degree ofadvancement of the wavefront can be determined by measuring pixelintensity and determining the point of image drop-off from highintensity (white) to low intensity (black).

The devices of the present technology can also be used to identifyparticular gas phase compounds. In some embodiments, the detectionregion of the device comprises an array of at least two differentmesogens. The pattern of response to the at least two different mesogenscan be used to identify particular compounds.

In some embodiments, gasoline vapor, or a component of gasoline vaporsuch as octane, is detected by phase transition of a liquid crystal. Insome preferred embodiments, devices for detecting gasoline vaporcomprise at least one surface in contact with a liquid crystalcomposition, wherein the liquid crystal composition. In someembodiments, the liquid crystal composition comprises a mesogen selectedfrom the group consisting of MBBA, EBBA, E7, MLC-6812, MLC 12200, 5CB(4-n-pentyl-4′-cyanobiphenyl), 8CB (4-cyano-4′octylbiphenyl) and4-(trans-4-heptylcyclohexyl)-aniline.

In some embodiments, the device is exposed to a sample suspected ofcontaining gasoline vapor, for example an atmospheric sample, or isexposed to an area suspected of containing or susceptible to containinggasoline vapor. The presence of gasoline vapor is indicated by a phasetransition of the liquid crystal in the device. The phase transition maybe observed visually or detected by a method such as measurement ofchange in optical anisotropy, magnetic anisotropy, dielectricanisotropy, and measurement of phase transition temperature.

Reflection-Based Probing

Some embodiments of devices according to the technology comprise aFabry-Perot filter. The present technology is not limited to aparticular mechanism. Indeed, an understanding of the mechanism of thepresent technology is not needed to practice the technology.Nevertheless, when electromagnetic radiation propagates through aninterface between two dielectric media it undergoes reflection at theinterface. If a dielectric material is sandwiched between two highlyreflecting mirrors forming a cavity, multiple reflection of radiationoccurs in the cavity. For a given thickness and dielectric properties ofthe cavity, the reflected electromagnetic radiation interferesconstructively and shows a maximum at a particular wavelength. Thewavelength at which the reflected radiation shows a peak in intensitydepends on the thickness of cavity and the dielectric property of thecavity. When the refractive index of the cavity changes, the wavelengthat which the maximum reflection occurs also changes. In someembodiments, the mirrors are functionalized with receptors or othermoieties that interact with the specific analyte and binding of thetarget induces an orientational transition of the LC and hence a changein the dielectric property of the cavity. This change in the dielectricconstant results in a shift in the wavelength at which the reflectedintensity is maximal. In some embodiments, the analyte interactsdirectly with the LC to effect a change in the dielectric properties ofthe cavity.

In some embodiments, the Fabry-Perot filter devices comprise a firstsurface (e.g., an interior surface) displaying one or more mesogencompositions. In some embodiments the mesogen composition comprises areactive moiety. In some embodiments, the surface is reflective. In someembodiments, the first surface is gold. In some embodiments, the gold isdeposited on a supporting substrate, such as glass or silicon. Othersuitable substrates are described in more detail above. In furtherembodiments, the devices comprise a second surface coated in areflective material, preferably gold. In some embodiments, the secondsurface also displays one or more mesogen composition. In someembodiments, the mesogen comprises a reactive moiety. In someembodiments, the first and second surfaces are configured opposite oneanother to form a chamber there between. Preferably, the chamber isfillable with a liquid crystal. Some mesogens that find use in formingthe liquid crystal are listed above and include, but are not limited to,E7, MLC-6812, MLC 12200, MBBA, EBBA, 5CB (4-n-pentyl-4′-cyanobiphenyl),and 8CB (4-cyano-4′octylbiphenyl).

In some embodiments, at least one mesogen composition is deposited orotherwise interacts with the first or second surfaces. In someembodiments the mesogen comprises a reactive moiety. The presenttechnology is not limited to any particular reactive moiety. Indeed, avariety of reactive moieties may be utilized, including, but not limitedto, organic functional groups such as amines, carboxylic acids, drugs,chelating agents, crown ethers, cyclodextrins, or a combination thereof,a biomolecule such as a protein, an antigen binding protein such as amonoclonal antibody, a polyclonal antibody, a chimeric antibody, ahumanized antibody, a Fab fragment, a single chain antibody, etc., apeptide, a nucleic acid (e.g., single nucleotides or nucleosides,oligonucleotides, polynucleotides, and single and higher-strandednucleic acids) or combinations thereof.

The present technology is not limited to any particular substrate shape.Indeed, a variety of substrate shapes are contemplated, including, butnot limited to, discs, cylinders, and spheres. Disc shaped devices arepreferably configured as described above, with a single planar surfacethat is overlaid with a liquid crystal. In some embodiments, the discshave a diameter of between about 0.1 mm to 10 cm, e.g., about 1 mm toabout 100 mm. In some embodiments, highly reflecting mirrors areprepared by depositing ˜500-nanometer thick gold films on clean glassslides (or plastic films) using an electron beam evaporator. In furtherembodiments, gold mirrors are layered with mesogens that provide areactive moiety. In some embodiments, glass fiber rods (e.g.,approximately 25-micron diameter) mixed in isopropanol are sprayeduniformly over one of the functionalized mirrors. These rods act asspaces defining the thickness of the dielectric cavity. An optical cellis fabricated forming a cavity between two reflecting mirrors. In someembodiments, the mirrors are glued together using UV curable adhesives.The cavity is then filled with a liquid crystal such as4-n-pentyl-4′-cyanobiphenyl (5CB). The present technology is not limitedto a particular mechanism of action. Indeed an understanding of themechanism of action is not necessary to practice the present technology.Nevertheless, without exposure to the target analyte, the liquidcrystals assume one phase (e.g., nematic). In some embodiments, themirror assembly is placed in the path of the light in a spectrometer.For the optimized thickness, a peak appears in the transmitted intensityat a particular wavelength determined by the ordinary refractive indexof the LC materials. In some embodiments, upon exposure to an analyte,the liquid crystal undergoes a phase transition (e.g., isotropic). Insome preferred embodiments, the device is placed in a light path. A peakappears at a wavelength that corresponds to the average refractive indexof the nematic phase. The shift in the peak position of the transmissionspectrum indicates a change in the refractive index of the cavity causedby the phase transition of the liquid crystal that is induced byinteraction of the analyte with mesogen and/or with the reactivemoieties of the mesogen on the surface.

In some embodiments, hollow polymer cylinders (about 100 to 1000 micronsin diameter, e.g., about 500 micron in diameter; about 1 mm to 1 cm inlength, e.g., about 5 mm in length) are first coated with a reflectivematerial such as gold. In some embodiments, the coating is from about 50to about 1000 nm in thickness, e.g., about 500 nm thick. A spacer isthen formed on the cylinder. In some embodiments, the spacer is fromabout 50 to about 200 microns in thickness, e.g., about 25 microns. Insome embodiments, the spacer comprises glass fiber rods with a diameterof 25 microns (such as from EM Industries) or plastic micropearls(spheres) of diameter 25 micron (such as from Sekesui Chemicals, HongKong). In some embodiments, these spacers are mixed in isopropyl alcoholand then sprayed onto the cylinders. A ˜25-micron sacrificial layer ofphotoresist is then coated to these cylinders. Examples of usefulphotoresist layers include, but are not limited to SU8 2010 fromMichochem. Another thin nanoporous layer of gold is deposited on top ofthe sacrificial layer. The gold film with nanopores is stronglyreflecting but allows small molecules to penetrate through it. Thesacrificial layer is then dissolved in acetone. The spacers in betweentwo gold surfaces act as supporting struts. These hollow cylinders arethen filled with a liquid crystal. In some embodiments, the firstsurface is spherical and a second surface and chamber are formed asdescribed for the cylinder embodiments.

In other embodiments, the devices of the present technology form arugate filter. Again, the present technology is not limited to aparticular mechanism. Indeed, an understanding of the mechanism of thepresent technology is not needed to practice the technology.Nevertheless, as the electromagnetic radiation propagates through anumber of interfaces between dielectric layers, multiple reflectionsoccur at each interface and a portion of the radiation is transmittedand a portion of it is reflected. If the dielectric constant of themedium exhibits sinusoidal variation, then the reflected intensity showsa peak in the reflected intensity at a wavelength that depends on theaverage dielectric constant and the amplitude of sinusoidal variation ofdielectric constant. The position of the reflected peak in theelectromagnetic spectrum shifts as the average refractive index of thesinusoidal variation changes. Accordingly, in some embodiments, asinusoidal variation in the dielectric property is created byfabricating porous silicon with sinusoidal porosity gradient along thedepth. See, e.g., Li et al. (2003) Science 299: 2045-47; Seals et al.(2002) J. Applied. Phys. 91(4): 2519-23; Schmedake et al. (2002) Adv.Mater. 14(18): 1270-72; Link et al. (2003) Proc. Nat'l Acad. Sci USA100(19): 10607-10, all of which are incorporated herein by reference intheir entirety. When the pores are filled with LCs, the LCs take on aspecific phase. Upon exposure to the target analyte the LC undergoes aphase transition, which induces a change in the dielectric constant ofthe pores resulting in a shift in the position of the peak.

The present technology is not limited to the use of any particular typeof silicon substrate. In some embodiments, the silicon substrate is ap-type, boron-doped silicon wafer with about a 1 mOhm-cm resistivity. Insome embodiments the silicon wafer is polished. In some embodiments, thesilicon substrate is ultrasonicated in isopropanol and then rinsed withwater. In some embodiments, the silicon wafers are etched using ananodization-etching process with a mixture of 48% hydrofluoric acid andabsolute ethanol (1:3 by volume) in a polytetrafluoroethylene (e.g.,“Teflon”) cell using a sinusoidally modulated current density togenerate a sinusoidal variation in the porosity gradient. In furtherembodiments, the amplitude, period, and duration of the sinusoidalcurrent density are adjusted to achieve the optimum porous size anddistribution. It will be recognized that these parameters can be variedand optimized for the detection of different analytes. In still furtherembodiments, the current density is then ramped up so that afreestanding film of the porous silicon is detached from the substrate.

In still further embodiments, devices such as those described above areirradiated with electromagnetic radiation from the radio frequencyregion, including, but not limited to, frequencies between 1 KHz and 10THz, and including the VLF, LF, MF, HF, VHF, UHF, SHF, and EHF regionsof the radio spectrum. Studies have demonstrated that analysis of thereflection and/or transmission spectra of RF radiation can be used toidentify analytes. See, e.g., U.S. Pat. Appl. 2004086929; Choi et al.,Intl. J High Speed Electronics and Systems 13(4): 937-950 (2003); vander Weide, Springer Series in Optical Sciences (2003), 85:3 17-334(2003), all of which are incorporated herein by reference. In someembodiments of the technology, a change in phase of a liquid crystalgives rise to a change in the reflection or transmission spectra of RFradiation. In further embodiments of the technology, the frequency ofthe radiation is in the 0.1 to 10 THz range. Methods known to thoseskilled in the art are used to analyze the radiation returned to adetector following interaction with the liquid crystal.

Photoluminescence

In some embodiments, a liquid crystal phase transition is detected byphotoluminescence. The present technology is not limited to a particularmechanism of action. Indeed, an understanding of the mechanism of actionis not necessary to understand the present technology. Nevertheless,when silicon with a nanometer scale porous structure is exposed toelectromagnetic radiation having a short wavelength, typically in theultraviolet region, electron-hole pairs are created. These excesscarriers subsequently recombine and emit electromagnetic radiation. Asthe characteristic size of the structures in the porous silicondecreases to the nanometer scale, the band gap of the siliconnanostructures progressively widens. The recombination of these quantumconfined carriers (electron-hole pair) in the wide band gap causesemission of electromagnetic radiation in the visible region. Thewavelength of the emitted light depends on the dielectric constant ofthe materials filling the pores and the structure of the poresthemselves. When the surfaces of the pores are filled with the liquidcrystal, the liquid crystal takes on one phase (e.g., nematic). Theporous silicon then emits light at a wavelength that corresponds to theradial distribution of the liquid crystal molecules. When the targetanalyte binds to the mesogen or to reactive moieties on the mesogensfilling the pores, the liquid crystal undergoes a phase transition(e.g., to isotropic) causing a change in the dielectric constant. Thisresults in a change in the position of the peak. It will be recognizedthat the present technology is not limited to any particular type ofchange in liquid crystal phase and that the described change fromnematic to isotropic is exemplary. Other changes are also contemplated,including, for example, from smectic to nematic or changes in the amountof twist, where, for example, cholesteric liquid crystals are utilized.

In some embodiments, porous silicon substrates are fabricated andfunctionalized as described above. In further embodiments, the poroussilicon is illuminated by a UV light. The exact wavelength of the UVlight depends on the actual pore size, pore size distribution, and therefractive index of the liquid crystal material. The photoluminescenceof the porous silicon is measured using a UV-visible spectrophotometer.The spectrum shows a peak at a wavelength corresponding to the phase ofthe liquid crystal. In some embodiments, the porous silicon is exposedto the target analyte in a closed chamber. The present technology is notlimited to any particular mechanism of action. Indeed, an understandingof the mechanism of action is not necessary to understand the presenttechnology. Nevertheless, as the target analyte binds to the mesogens orto a reactive moiety provided by the mesogen on the surface of thepores, the LC undergoes a phase transition. It is contemplated that thechange in the phase of the liquid crystal corresponds to a change in thespectrum of radiation emitted by the porous silicon.

Fluorescence Based Detection

In some embodiments, detection is accomplished using a fluorescentreporter system. The present technology is not limited to any particularmechanism of action. Indeed, an understanding of the mechanism of actionis not necessary to understand the present technology. Nevertheless,certain compounds such as4-(4-dihexadecylsminostyryl)-N-methylpyridinium iodide (DIA);1,3,5,7,8-pentamethyl-2,6,-di-t-butylpyrromethane-difluoreborate(PM-597); 4-(dicynaomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM); eurobium(III) thenoyltrifluoroacetonatetrihydrate (Eu(TTA)3.H2O); etc., when dissolved in a liquid crystal emitvisible light upon exposure to UV radiation. The intensity and thewavelength of the emitted light depend on the orientation of the dyemolecules with respect to liquid crystal phase. If the dye molecules areimmobilized on a surface in a fixed orientation with respect to thesurface and the liquid crystal undergoes a phase transition, thecharacteristics of the emitted radiation change. When an analyteinteracts with the LC (e.g., directly or binds to a moiety such asreceptor on the LC mesogens), the liquid crystal undergoes phasetransition and the wavelength of the emitted light changes.

Accordingly, in some embodiments, a thin gold film is deposited on asubstrate (e.g., a UV-transparent quartz substrate or a plastic film)using an electron beam evaporator. The gold surface is assembled into aliquid crystal assay device using small glass spacer rods as describedabove. The device is then filled with a liquid crystal. In someembodiments, the LC provides a reactive moiety. The present technologyis not limited to any particular mechanism of action. Indeed, anunderstanding of the mechanism of action is not necessary to understandthe present technology. Nevertheless, without exposure to the targetanalyte, the liquid crystal takes on one phase, for example, nematic. Insome embodiments, the optical cell is irradiated with UV light, which insome embodiments is provided by a laser. In the absence of the analyte,the fluorescent molecules emit visible light at a wavelength thatcorresponds to the nematic phase of liquid crystals. When the device isexposed to an analyte the liquid crystal undergoes a phase transitionto, for example, an isotropic phase. The shift in the peak position ofthe fluorescence spectrum (e.g., a change in the color of the emittedlight) indicates a change in the dielectric environment of thefluorescent molecules. This change is caused by the phase transition ofthe LC induced by the analyte interacting with the mesogens or bybinding of the analyte to a reactive moiety on the mesogen.

In other embodiments of the technology, fluorescent dye molecules suchas Acridine Orange Base; Rhodamine 6G; perchlorate;5-decyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene-3-propionic acid;Nile Red;N,N′-Bis(2,5-di-tert-butylphenyl)-3,4,9,10-perylenedicarboximide; etc.,are dissolved into the liquid crystal forming a guest-host system. Thepresent technology is not limited to any particular mechanism of action.Indeed, an understanding of the mechanism of action is not necessary tounderstand the present technology. Nevertheless, the orientation of thedye molecule, in general, is dependent on the phase of the LC in the LCcell. When a beam of light (typically in the visible region) having apolarization parallel to the transition dipole moment of the dye, ispassed through the guest host system, the incident light energy isabsorbed by the dye molecule. The dye molecules then emit (e.g.,visible) light at a different wavelength. However, if the incident lightis polarized perpendicularly to the transition dipole moment of the dyemolecule, it is not absorbed and the dye molecules do not emit anyradiation. Therefore, in the absence of the target analyte, when theguest-host system is interrogated by a polarized light corresponding tothe excitation wavelength of the dye used, the light emitted from thesystem is composed of the excitation wavelength. If the analyte ispresent in the ambient, it interacts with the LC or the functionalizedsurface and the liquid crystal undergoes phase transition, e.g., fromthe nematic to the isotropic phase. This causes a rotation of thetransition moment of the dye molecule parallel to the polarizationdirection of the excitation beam. The dye molecules then absorb theincident wavelength and emit light at different wavelength. Thus, byprobing a liquid crystal-dye mixture using polarized light propagatingperpendicularly to the cell surface, the presence of the analyte in theenvironment can be probed. The liquid crystal assay device cell isfabricated as described above except that it is filled with a liquidcrystal-dye mixture. For the interrogation, the polarization can beintegrated on the liquid crystal cell or can be probed by sendingpolarized light.

In still further embodiments, the fluorescent properties of quantum dotsare utilized for detecting analytes. The present technology is notlimited to any particular mechanism of action. Indeed, an understandingof the mechanism of action is not necessary to understand the presenttechnology. Nevertheless, some semiconductor quantum dots (e.g., withnanometer size) emit visible light when exposed to UV radiation. Due toquantum confinement, the electron-hole pairs trapped at the surface havea large band gap. Because of this large band gap, these semiconductorquantum dots absorb light in the UV region. The wavelength of lightemitted by these fluorescence particles depends on their size and onproperties of the surrounding medium, such as but not limited to, thedielectric constant of the surrounding medium. In some embodiments,these quantum dots are functionalized with a receptor targeted for theanalyte so that a liquid crystal in contact with them assumes anorientation perpendicular to the surface of the quantum dots. Uponirradiation from a UV light source, the fluorescent spectrum shows apeak at a particular wavelength. When these dots are exposed to theanalyte, the liquid crystal undergoesa phase transition, which causes ashift in the peak position.

The present technology is not limited to the use of any particular typeof quantum dot. In some preferred embodiments, cadmium selenide quantumdots with thin zinc sulfide and polymer coatings are functionalized witha carboxylic acid terminated organic layer (for example11-mercaptoundecanoic acid) and then treated to display a reactivemoiety as described above (e.g., aniline-like groups). In some preferredembodiments, the quantum dots are dispersed in a liquid crystal (e.g.,5CB). The functionalized quantum dots influence the phase of the liquidcrystals (nematic). In further preferred embodiments, a liquid crystalassay device is fabricated by forming a cavity (preferably 5 to 100microns, most preferably about 25 microns) between two untreated UVtransparent quartz substrates. The cavity between the substrates isfilled with the mixture of functionalized quantum dots and liquidcrystal. In still further preferred embodiments, the optical cell isexposed to an analyte (such as nitric oxide or nitrogen dioxide) andthen probed with a UV light source, such as a laser. When the analytebinds to the receptors on the quantum dots, the liquid crystal undergoesa phase transition to isotropic, disrupting the quantum dots and therebyaffecting the color of light emitted by them.

Electrical Detection

In some embodiments, a change in the physical properties of an LC isdetected by measuring a change in the dielectric constant of the liquidcrystals that results from an interaction of the analyte with the LC.

Sub-Responsive Exposure to Analyte

In some embodiments, devices are exposed to a “sub-responsive” amount ofan analyte prior to its use to detect the analyte (e.g., prior toexposing the device to a sample comprising or suspected of comprisingthe analyte). In these embodiments, the device will demonstrate aresponse to a lower amount of analyte in a test sample than a devicethat has not been exposed to a sub-responsive amount of the analyte. Asused herein, a “sub-responsive” amount, concentration, mass, etc. of ananalyte is an amount, concentration, mass, etc. of the analyte thatreacts with the device but that does not cause a detectable response(e.g., signal) from the device. Exposure to a sub-responsive amount ofan analyte thus “pushes” or “primes” the device to demonstrate aresponse to a small amount of analyte.

IV Analytes

The methods and devices of the present technology can be used to detecta variety of analytes in the gas phase. The present technology is notlimited to the detection of any particular type of analyte. Exemplaryanalytes include, but are not limited to, sulfur compounds, nitrogencompounds, thiols, alcohols, acids, oxides, alkanes, alkenes, alkynes,and phosphates.

The present technology finds use in the detection of variety of sulfurcompounds. In some embodiments, the sulfur compounds are from a groupthat includes sulfides, disulfides, sulfites or sulfates, including butnot limited to hydrogen sulfide, Chloromethyl trifluoromethyl sulfide,Ethylene sulfide, Dimethyl sulfide, Methyl Sulfide, Propylene sulfide,Trimethylene sulfide, 2-Chloroethyl methyl sulfide,2-(Methylthio)ethanol, Ethyl methyl sulfide, Bis(methylthio)methane,2-(Methylthio)ethylamine, N-Methyl-1-(methylthio)-2-nitroethenamine,Allyl methyl sulfide, 2-Chloroethyl ethyl sulfide,3-(Methylthio)-1-propanol, 2,2′-Thiodiethanol, 2,2′-Dithiodiethanol,Diethyl sulfide, Methyl propyl disulfide, Tris(methylthio)methane,2-(Ethylthio)ethylamine, 3-(Methylthio)propylamine, Cystaminedihydrochloride, 4-(Methylthio)-1-butanol, tert-Butyl methyl sulfide,Cyclohexene sulfide, Diallyl sulfide, Allyl disulfide,3,3′-Thiodipropanol, 3,3′-Thiodipropanol, 3,6-Dithia-1,8-octanediol,Dipropyl sulfide, Isopropyl sulfide, Dipropyl disulfide, Isopropyldisulfide, 4-(Trifluoromethylthio)bromobenzene,4-(Trifluoromethylthio)phenol, Phenyl trifluoromethyl sulfide,3,5-Dichlorothioanisole, Chloromethyl 4-chlorophenyl sulfide,4-(Trifluoromethylthio)aniline, 2-Bromothioanisole, 3-Bromothioanisole,4-Bromothioanisole, 2-Chlorothioanisole, 3-Chlorothioanisole,4-Chlorothioanisole, Chloromethyl phenyl sulfide, 2-Fluorothioanisole,4-Fluorothioanisole, 4-Nitrothioanisole, Thioanisole,2-(Methylthio)aniline, 3-(Methylthio)aniline, 4-(Methylthio)aniline,2-(Methylthio)cyclohexanone, 3-(Methylthio)-1-hexanol,4-(Trifluoromethylthio)benzyl bromide, 4-(Trifluoromethylthio)benzylalcohol, Phenyl vinyl sulfide, 4-(Methylthio)benzyl bromide,2-Chloroethyl phenyl sulfide, 4-(Methylthio)benzyl chloride,2-Methoxythioanisole, 2-(Phenylthio)ethanol, 4-Methoxythioanisole,4-(Methylthio)benzyl alcohol, Methoxymethyl phenyl sulfide, Ethyl phenylsulfide, Methyl p-tolyl sulfide, Dibutyl sulfide, Dibutyl disulfide,Bis(trimethylsilylmethyl) sulfide, Phenyl propargyl sulfide,(4-Chlorophenylthio)acetone, Benzyl 2,2,2-trifluoroethyl sulfide,4′-(Methylthio)acetophenone, Allyl phenyl sulfide, Cyclopropyl phenylsulfide, 2-Nitro-5-(propylthio)aniline, S-Benzylcysteaminehydrochloride, Isoamyl sulfide, 4′-Methylthioisobutyrophenone,Pentafluorophenyl sulfide, Bithionol, Bis(3,5-dichlorophenyl) disulfide,Bis(3,5-dichlorophenyl) disulfide, Bis(4-chlorophenyl) disulfide,3-Nitrophenyl disulfide, 4-Nitrophenyl disulfide, Bis(2-nitrophenyl)disulfide, 2-Nitrophenyl phenyl sulfide, 4-Nitrophenyl phenyl sulfide,2-(4-Chlorophenylthio)aniline, 4-Amino-4′-nitrodiphenyl sulfide,3,3′-Dihydroxydiphenyl disulfide, Diphenyl sulfide, Diphenyl disulfide,Phenyl disulfide, 2-(Phenylthio)aniline, 2,2′-Diaminophenylsulfide,4,4′-Diaminodiphenyl sulfide, 2,2′-Dithiodianiline, Hexyl sulfide,Benzyl phenyl sulfide, Bis(phenylthio)methane, Dodecyl methyl sulfide,2-Nitro-p-tolyl disulfide, Bis(4-methoxyphenyl) disulfide, Dibenzylsulfide, Dibenzyl disulfide, p-Tolyl disulfide, Benzyl trisulfide,2-[2-(Aminomethyl)phenylthiolbenzyl alcohol, Phenylacetyl disulfide,Dioctyl sulfide, Chlorotriphenylmethyl disulfide,Tris(phenylthio)methane, Tris(phenylthio)methane, Dodecyl sulfide,Hexakis[(4-methylphenyl)thio]benzene, and Hexakis(benzylthio)benzene,Potassium methyl sulfate, Formaldehyde-sodium bisulfite adduct, Methylsulfate sodium salt, Glyoxal bis(sodium hydrogen sulfite) adducthydrate, Ethylene sulfite, Glyoxal sodium bisulfite addition compoundhydrate, Dimethyl sulfite, Diethyl sulfite, Glutaraldehyde sodiumbisulfite addition compound, Dipropyl sulfate, 4-Acetylphenyl sulfatepotassium salt, Sodium 2-ethylhexyl sulfate, Sodium octyl sulfate,Dibutyl sulfate, 4-Hydroxy-3-methoxyphenylglycol sulfate potassium salt,Sodium dodecyl sulfate, Ammonium lauryl sulfate solution, Tetradecylsulfate sodium salt, and Octadecyl sulfate sodium salt. In someembodiments the sulfur compounds are from a group that includestriflates such as but limited to (Trimethylsilyl)methyltrifluoromethanesulfonate, (Trimethylsilyl)methyltrifluoromethanesulfonate, 4-Nitrophenyl trifluoromethanesulfonate,Phenyl trifluoromethanesulfonate, 1-Cyclohexenyltrifluoromethanesulfonate, Catechol bis(trifluoromethanesulfonate),p-Tolyl trifluoromethanesulfonate, 4-Acetylphenyltrifluoromethanesulfonate, 2,6-Dimethoxyphenyltrifluoromethanesulfonate, 3,5-Dimethoxyphenyltrifluoromethanesulfonate, 2-(Trimethylsilyl)phenyltrifluoromethanesulfonate,Di-tert-butylsilylbis(trifluoromethanesulfonate), 1-Naphthyltrifluoromethanesulfonate, 2-Naphthyl trifluoromethanesulfonate,4,4′-Biphenol bis(trifluoromethanesulfonate), 3,5-Di-tert-butylphenyltrifluoromethanesulfonate, 1,1′-Bi-2-naphtholbis(trifluoromethanesulfonate).

In some embodiments, the sulfur is in an oxidized state, including butnot limited to sulfur dioxide, sulfur trioxide, sulfuric acid, sulfuroxide, Methyl phenyl sulfoxide, Phenyl vinyl sulfoxide, Methyl p-tolylsulfoxide, Butyl sulfoxide, Methyl 2-phenylsulfinylacetate, Diphenylsulfoxide, p-Tolyl sulfoxide, Dodecyl methyl sulfoxide, and Dibenzylsulfoxide. In other embodiments, the sulfur is in a compound withhalogenated elements, such as sulfenyl halides, sulfinyl halides, andsulfonyl halides including but not limited to Chlorocarbonylsulfenylchloride, Methoxycarbonylsulfenyl chloride, 2,4-Dinitrobenzenesulfenylchloride, 4-Nitrobenzenesulfenyl chloride, Trichloromethanesulfinylchloride, tert-Butylsulfinyl chloride, 2,4,5-Trichlorobenzenesulfonylchloride, 3, 4-Dichlorobenzylsulfonyl chloride, 2-Chlorobenzylsulfonylchloride, Trichloromethanesulfonyl chloride, Methanesulfonyl fluoride,Chlorosulfonylacetyl chloride, N,N-Dimethylsulfamoyl chloride,Cyclopropanesulfonyl chloride, 2-Propanesulfonyl chloride,Perfluoro-1-butanesulfonyl fluoride, 2-Bromo-4,6-difluorobenzenesulfonylchloride, 2,3,4-Trichlorobenzenesulfonyl chloride,2,5-Dibromobenzenesulfonyl chloride, Benzene-1,3-disulfonyl chloride,Cyclohexanesulfonyl chloride, m-Toluenesulfonyl chloride, disulfurdichloride, sulfur hexafluoride, thionyl chloride, and sulfurylchloride.

In some embodiments, the gas compound contains nitrogen, including butnot limited to nitrogen, ammonia, 1,3,5-Trinitrobenzene(TNB), Methylnitrate, Nitroglycerin (NG), Triaminotrinitrobenzene (TATB), andPentaerythritol tetranitrate (PETN). In some embodiments, the nitrogencontaining compound is an amine. The amine may have an alkyl or an arylfunctional group, may be aliphatic or aromatic in structure, may berepresented by an organic compound that is a primary, secondary ortertiary amine including but not limited to methylamine, ethanolamine,trisamine, dimethylamine, methylethanolamine, aziridine, azetidine,pyrrolidine, piperidine, trimethylamine, dimethylethanolamine, aniline,cadaverine, idole, putrescine, and bis-tris methane.

In some embodiments, the gas compound is a thiol, including but notlimited to methanethiol, ethanethiol, cysteine, 2-mercaptoethanol,dithiothreitol, and 2-mercaptoindole.

In some embodiments, the gas compound is an alcohol. The alcohol may becyclic or acyclic, may be represented by an organic compound that is aprimary, secondary or tertiary alcohol including but not limited tomethanol, ethanol, isopropanol, tert-butyl alcohol, propanol,cyclopropanols, cyclobutanols, cyclopentanols, cyclopropanols,cyclohexanol, cycloheptanols, benzylic alcohols, diarylmethanols, andallylic alcohols.

In some embodiments, the gas compound is an acid. The acid may beorganic or inorganic, monoprotic, diprotic or triprotic, including butnot limited to acetic acid, sulfuric acid, hydrochloric acid,hypochlorous acid, chorous acid, chloric acid, perchloric acid,hydrobromic acid, hydroiodic acid, hydrofluoric acid, nitric acid,nitrous acid, carbonic acid, phosphoric acid, citric acid, formic acid,chromic acid, methanesulfonic acid, ethanesulfonic acid, benzenesulfonicacid, toluenesulfonic acid, folic acid, and salicylic acid.

In some embodiments, the gas compound is an oxide or its derivative,including but not limited to oxygen, nitric oxide, nitrous oxide,nitrogen dioxide, nitrogen dioxide, carbon monoxide, carbon dioxide,sulfur dioxide, ozone, and peroxides.

In some embodiments, the gas compounds are phosphates that may beorganic or inorganic, including but not limited to ammonium phosphate,boranophosphate, diammonium phosphate, phosphagen, phosphate, phosphoricacid, phosphotungstic acid, polyphosphate, pyrophosphoric acid, and ureaphosphate. In some embodiments, the organophosphates are those used aspesticides, including, but not limited to, Acephate (Orthene),Azinphos-ethyl, Azinphos-methyl (Guthion), Azinphos-methyl oxon,Bromophos-methyl, Carbophenothion (Trithion), Chlorfenvinphos (Supona),Chloropyrifos (Dursban/Lorsban), Chlorpyrifos-methyl, Chlorthiophos,Coumaphos (Co-Ral), Crotoxyphos (Ciodrin), Cyanophos, DEF (Buffos),Demeton (Systox), Demeton-Dialifor (Torok), Diazinon (O Analog),Diazinon (Spectracide), Dichlorvos-DDVP (Vapona), Dicrotophos (Bidrin),Dimethoate (Cygon), Dioxathion (Delnav), Disulfoton (Disyston),Disulfoton Sulfone, Edifenphos, EPN, Ethion (Nialate), Ethoprop (Mocap),Ethyl Parathion, Fenamiphos (Nemacur), Fenitrothion (Sumithion),Fensulfothion (Dasanit), Fenthion (Baytex), Fonofos (Dyfonate),Formothion, Heptenophos, Imidan (Phosmet), Isazophos (Triumph),Isofenphos (Amaze), Leptophos (Phosvel), Malaoxon, Malathion (Celthion),Merphos (Tribufos), Methamidophos (Monitor 4), Methidathion, MethylParathion (Metacide), Mevinphos (Phosdrin), Monocrotophos, Naled,Omethoate (Dimethoate O analog), Parathion (Alkron), Paroxon, Phorate(Thimet), Phorate-o, Phorate Sulfone, Phorate Sulfoxide, Phosalone,Phosphamidon (Dimecron), Piperophos, Pirimiphos-ethyl,Pirimiphos-methyl, Profenofos (Curacron), Propetamphos (Safrotin),Pyrazophos (Afgan), Quinalphos, Ronnel (Ectoral) (Fenchlorphos),Sulprofos (Bolstar), Terbufos (Counter), Tetrachlorvinphos (Gardona),Thionazin (Zinophos), and Triazophos (Hostathion). In some embodiments,the organophosphates are nerve agents (e.g., agents of war), including,but not limited to G agents (GD, soman; GB, sarin; and GA, tabun) andthe V agents (VX).

In some embodiments, the analyte comprises an alkane (e.g., a paraffin),e.g., an alkane comprising between 4 and 12 carbon atoms per molecule(commonly referred to as C4-C12, e.g., butane, pentane, hexane, heptane,octane, nonane, decane, undecane, dodecane). In some embodiments, theanalyte comprises an n-alkane and in some embodiments, the analytescomprises a branched alkane. In some embodiments, the analyte comprisesa cycloalkane and/or a naphthene. In some embodiments, the analytescomprises an alkene (e.g., olefin), cycloalkene, isoalkane, aromatic(e.g., benzene, toluene, xylene, ethylbenzene, C3-benzene, C4-benzene),and/or an alkyne.

In some embodiments, the analyte comprises a mixture of organiccompounds that is known by the general name of “gasoline”, “petrol”,“casing head gasoline”, “motor fuel”, or “motor spirit”. Accordingly, insome embodiments, the analytes is or comprises an additive and/orblending agent such as an anti-knock agent, anti-oxidant, metaldeactivator, lead scavenger, anti-rust agent, anti-icing agent,upper-cylinder lubricant, detergent, and/or a dye.

V Reactive Moieties

A wide variety of chemical sensors can be fabricated that will detecttrace chemical vapors utilizing the interactions between liquid crystalsand the analyte. The physical (e.g., optical and electrical) and thealignment properties of liquid crystal are governed by theintermolecular interactions of its functional moieties where a chemicalchange in the liquid crystal moiety is likely to alter its properties.Liquid crystal has the ability to influence the rates and energetics oforganic reactions due to its integrated molecular arrangements.Incorporating a functional moiety that reacts with the target analytecan affect a change in liquid crystal molecules that will be translatedinto its observed properties.

The present technology provides a method for the detection ordifferentiation and quantitative measurement of a wide range of chemicalvapors, such as oxides of nitrogen, ozone, amines, alcohols, thiols,etc. The liquid crystal can be tuned or functionalized by a combinationof processes, such as, liquid crystals having reactive organicfunctional groups (—OH, —C═C—, —C≡C—, —N═N—, —NH2, —COOH, etc.),metal-ligand interaction, metal-liquid crystal interaction,metal-ligand-liquid crystal interaction. The choice of a particularliquid crystal composition will be based on the analyte that interactswith LC either by chemical reaction, metal-ligand coordinationinteraction, or dipole-dipole interactions (e.g., by changes in thepolarity of the LC environment) that fulfills the requirements: (i) thetarget vapors should interact strongly with the LC, and (ii) thisinteraction must be coupled to a change in the LC.

The interaction between the analyte and the LC will be dependent on theanalyte of interest and the active functional group present in the LC.This particular detection mechanism will involve acid—base,oxidation-reduction, substitution reaction, or combinations thereof atthe functionalized moiety in the liquid crystal. The interaction of thetarget analytes with the liquid crystals will manifest as a change inthe physical properties of liquid crystals (phase transition, opticalbirefringence, dielectric anisotropy, magnetic isotropy, or change inthe orientation of liquid crystals on a surface) that can be detectedusing a variety of instruments capable of detecting these physicalchanges.

A variety of reactive moieties find use in the present technology. Insome embodiments, the reactive moieties are functional groups availableon the liquid crystal that is overlaid on a substrate. In someembodiments, a substrate is overlaid with a thin film of a solution toprovide the reactive groups on the surfaces of the substrate.

VI Substrates

Substrates that find use in practicing the present technology can bemade of practically any physicochemically stable material. In apreferred embodiment, the substrate material is non-reactive towards theconstituents of the mesogenic layer. The substrates can be either rigidor flexible and can be either optically transparent or optically opaque.The substrates can be electrical insulators, conductors orsemiconductors. Further, the substrates can be substantially impermeableto liquids, vapors and/or gases or, alternatively, the substrates can bepermeable to one or more of these classes of materials. Exemplarysubstrate materials include, but are not limited to, inorganic crystals,inorganic glasses, organic glasses, inorganic oxides, metals, metaloxides, semiconductors, conductors, salts, organic polymers andcombinations thereof. In some embodiments, the substrates havemicropillared features thereon for the stabilization of the liquidcrystal overlay and/or other reagents to the substrate surface ordetection regions thereon.

Inorganic Crystal and Glasses

In some embodiments of the present technology, inorganic crystals andinorganic glasses are utilized as substrate materials (e.g., LiF, NaF,NaCl, KBr, KI, CaF₂, MgF₂, HgF₂, BN, AsS₃, ZnS, Si₃N₄, and the like).The crystals and glasses can be prepared by conventional techniques(see, e.g., Goodman., Crystal Growth Theory and Techniques, PlenumPress, New York 1974). Alternatively, the crystals can be purchasedcommercially (e.g., Fisher Scientific). The crystals can be the solecomponent of the substrate or they can be coated with one or moreadditional substrate components. Thus, it is within the scope of thepresent technology to utilize crystals coated with, for example, anorganic polymer. Additionally, a crystal can constitute a portion of asubstrate that contacts another portion of the substrate made of adifferent material, or a different physical form (e.g., a glass) of thesame material. Other useful substrate configurations utilizing inorganiccrystals and/or glasses will be apparent to those of skill in the art.

Inorganic Oxides

In other embodiments of the present technology, inorganic oxides areutilized as the substrate. Inorganic oxides of use in the presenttechnology include, for example, Cs₂O, Mg(OH)₂, TiO₂, ZrO₂, CeO₂, Y₂O₃,Cr₂O₃, Fe₂O₃, NiO, ZnO, Al₂O₃, SiO₂ (glass), quartz, In₂O₃, SO₂, PbO₂,and the like. The inorganic oxides can be utilized in a variety ofphysical forms such as films, supported powders, glasses, crystals, andthe like. A substrate can consist of a single inorganic oxide or acomposite of more than one inorganic oxide. For example, a composite ofinorganic oxides can have a layered structure (e.g., a second oxidedeposited on a first oxide) or two or more oxides can be arranged in acontiguous non-layered structure. In addition, one or more oxides can beadmixed as particles of various sizes and deposited on a support such asa glass or metal sheet. Further, a layer of one or more inorganic oxidescan be intercalated between two other substrate layers (e.g., metaloxide metal, metal oxide-crystal).

In some embodiments, the substrate is a rigid structure that isimpermeable to liquids and gases. In this embodiment, the substrateconsists of a glass plate onto which a metal, such as gold, is layeredby evaporative deposition. In a still further embodiment, the substrateis a glass plate (SiO₂) onto which a first metal layer such as titaniumor gold has been layered. A layer of a second metal (e.g., gold) is thenlayered on top of the first metal layer (e.g., titanium).

Organic Polymers and Glasses

In still other embodiments of the present technology, organic polymersare utilized as substrate materials. Organic polymers useful assubstrates in the present technology include polymers that are permeableto gases, liquids, and molecules in solution. Other useful polymers arethose that are impermeable to one or more of these same classes ofcompounds. Many of these polymers can be prepared as glasses.

Organic polymers that form useful substrates include, for example,polyalkenes (e.g., polyethylene, polyisobutene, polybutadiene),polyacrylics (e.g., polyacrylate, polymethyl methacrylate,polycyanoacrylate), polyvinyls (e.g., polyvinyl alcohol, polyvinylacetate, polyvinyl butyral, polyvinyl chloride), polystyrenes,polycarbonates, polyesters, polyurethanes, polyamides, polyimide s,polysulfone, polysiloxanes, polyheterocycles, cellulose derivatives(e.g., methyl cellulose, cellulose acetate, nitrocellulose),polysilanes, fluorinated polymers, epoxies, polyethers, and phenolicresins (see, Cognard (1982) “Alignment of Nematic Liquid Crystals andTheir Mixtures” in Mol. Cryst. Liq. Cryst. 1: 174). Some organicpolymers include polydimethylsiloxane, polyethylene, polyacrylonitrile,cellulosic materials, polycarbonates, and polyvinyl pyridinium.

In some embodiments, the substrate is permeable and it comprises a layerof gold, or gold over titanium, which is deposited on a polymericmembrane, or other material, that is permeable to liquids, vapors,and/or gases. The liquids and gases can be pure compounds (e.g.,chloroform, carbon monoxide) or they can be compounds that are dispersedin other molecules (e.g., aqueous protein solutions, herbicides in air,alcoholic solutions of small organic molecules, etc.). Useful permeablemembranes include, but are not limited to, flexible cellulosic materials(e.g., regenerated cellulose dialysis membranes), rigid cellulosicmaterials (e.g., cellulose ester dialysis membranes), rigidpolyvinylidene fluoride membranes, polydimethylsiloxane, and tracketched polycarbonate membranes.

In a further embodiment, a layer of gold on the permeable membrane isitself permeable. In some embodiments, the permeable gold layer has athickness of about 70 Angstroms or less.

In those embodiments wherein the permeability of the substrate is not aconcern and a layer of a metal film is used, the film can be as thick asis necessary for a particular application. For example, if the film isused as an electrode, the film can be thicker than in an embodiment inwhich it is necessary for the film to be transparent or semi-transparentto light.

Thus, in some embodiments, the film has a thickness from about 0.01nanometer to about 1 micrometer, e.g., about 5 nanometers to about 100nanometers. In some embodiments, the film has a thickness of from about10 nanometers to about 50 nanometers.

Arrays

In some embodiments, the LC composition comprising reactive moieties isarrayed on the substrates using stamping, microcontact printing, orink-jet printing. In still further embodiments, reactive moieties arespotted onto a suitable substrate. Such spotting can be done by handwith a capillary tube or a micropipette, or by an automated spottingapparatus such as those available from Affymetrix and Gilson (see, e.g.,U.S. Pat. Nos. 5,601,980; 6,242,266; 6,040,193; and 5,700,637; each ofwhich is incorporated herein by reference).

Micro-Structured Features

In some embodiments, the substrates utilized in the devices of thepresent technology comprise one more micro-structured features. In someembodiments, micro-structured features on the substrate augment thespreading of the liquid crystal composition. In still other embodiments,the micro-structured features stabilize the liquid crystal overlayand/or other reagents on the substrate surface or detection regionsthereon. In a paper by Frisk et al (2006, Lab on a Chip 6: 1504), liquidwas dispensed onto a micromachined biosensor substrate that wassuspended vertically and remained stably dispersed (and immune togravitational forces and shock) on that substrate. Following on thisresult, Sridharamurthy et al (2008, Smart Mater Struct 17) demonstratedthat microstructures could be used to support a film of liquid crystal.In contrast to these systems, in some preferred embodiments, themicro-structured features are made my depositing a polymer on thesubstrate and etching away areas between the micro-structured featuresor made from the same material as the substrate. Additionally, in someembodiments, the analyte interacts and/or reacts with the LC compositionrather than competing at the surface of the substrate.

Accordingly, in some embodiments, the micro-features pattern the surfaceand are selected from the group consisting of a grid, a channel, aplurality of pillars, or an array of assay areas, or combinationthereof. In some embodiments, the micro-features are pillars thatproject from the surface of the substrates. In some embodiments, thesubstrates are comprised of glass, silicon, polymer, or a combinationthereof. In still further embodiments, the pillars are comprised of thesame material as the surface. In other embodiments, the pillars arecomprised of a different material than the surface. In some embodiments,the substrate is glass while the pillars are made from a polymericmaterial. The pillars may comprise a shape selected from the groupconsisting of circular, triangular, square, hexagonal, or a combinationthereof. The dimension of the pillars could be a variety of heights,widths, and spacings. Indeed, the pillar height may range from 1 micronto 50 microns, the width from 1 micron to 200 microns, and the spacingbetween pillars may range from 1 micron to 200 microns.

VII Mesogens

Any compound or mixture of compounds that forms a mesogenic layer can beused in conjunction with the present technology. The mesogens can formthermotropic, lyotropic, metallotropic, or cholesteric liquid crystals.The thermotropic, lyotropic, metallotropic, and cholesteric liquidcrystals can exist in a number of forms including nematic, isotropic,chiral nematic, smectic, polar smectic, chiral smectic, frustratedphases, and discotic phases.

Some mesogens that find use in embodiments of the technology aredisplayed in Table 2. In some embodiments, the mesogen is 5CB(4-pentyl-4′-cyanobiphenyl), MLC-6812, MLC 12200, MBBA, EBBA, or 8CB(4-cyano-4′-octylbiphenyl), and combinations thereof.

The mesogenic layer can be a substantially pure compound, or it cancontain other compounds, so called dopants, that enhance or altercharacteristics of the mesogen. Thus, in some embodiments, the mesogeniclayer further comprises a second compound, for example an alkane, whichexpands the temperature range over which the nematic and isotropicphases exist. Use of devices having mesogenic layers of this compositionallows for detection of the analyte reactive moiety interaction over agreater temperature range.

In some embodiments, the mesogenic layer further comprises a dichroicdye or a fluorescent compound. Examples of dichroic dyes and fluorescentcompounds useful in the present technology include, but are not limitedto, azobenzene, BTBP, polyazo compounds, anthraquinone, perylene dyes,and the like. In some embodiments, a dichroic dye of a fluorescentcompound is selected that complements the orientation dependence of theliquid crystal so that polarized light is not required for the assay. Insome embodiments, if the absorbance of the liquid crystal is in thevisible range, then phase changes can be observed using ambient lightwithout crossed polarizers. In some embodiments, the dichroic dye orfluorescent compound is used in combination with a fluorimeter andchanges in fluorescence are used to detect changes in phase transitionof the liquid crystal.

TABLE 2 Molecular structures of mesogens suitable for use in embodimentsof liquid crystal assay devices Mesogen Structure Anisaldazine

NCB

CBOOA

Comp A

Comp B

DB₇NO₂

DOBAMBC

nOm n = 1, m = 4: MBBA n = 2, m = 4: EBBA

nOBA n = 8: OOBA n = 9: NOBA

nmOBC

nOCB

nOSI

98P

PAA

PYP906

nSm

Although the disclosure herein refers to certain illustratedembodiments, it is to be understood that these embodiments are presentedby way of example and not by way of limitation.

EXAMPLES

The following examples are provided to demonstrate and furtherillustrate certain embodiments and aspects of the present technology andare not to be construed as limiting the scope thereof.

In the experimental disclosure that follows, the following abbreviationsare used: eq. (equivalents); M (molar); μM (micromolar); N (Normal); mol(moles); mmol (millimoles); μmol (micromoles); nmol (nanomoles); g(grams); mg (milligrams); μg (micrograms); ng (nanograms); 1 or L(liters); ml (milliliters); μl (microliters); cm (centimeters); mm(millimeters); μm (micrometers); nm (nanometers); C (degreesCentigrade); U (units), mU (milliunits); min. (minutes); sec. (seconds).

Example 1 Comparison Between Microfluidic Cells and Sandwich Cells

During the development of embodiments of the technology provided herein,data were collected from testing traditional sandwich cells (LC cellswhere all the space between two surfaces was filled with LC),microfluidic cells with small discrete sensing areas (i.e. LC cells witha headspace between a substrate with discrete sensing areas and topsurface) and microfluidic cells with large single sensing area (i.e. LCcells with a headspace between a substrate with large sensing areas andtop surface). Some experiments were also performed with varyingthicknesses of LC between the two sides of the sandwich cell and alsovarying thicknesses of the head space in microfluidic cells to accessthe effect of thickness in the response.

A traditional sandwich LC cell was prepared first by coating a 1″×3″aluminosilicate (AlSi) glass slide with 20 Å thick titanium layerfollowed by 100 Å thick gold film The gold coated slides were chemicallyfunctionalized by forming self-assembled monolayer of11-mercaptoundecanoic acid (MUA) after incubating the slide in 1 mMethanolic solution for ˜16 hrs. These slides were then rinsed thoroughlywith ethanol and dried in a stream of nitrogen (N₂). The slides werethen briefly (15 s) subjected to UV ozone treatment, then rinsedthoroughly with ethanol and dried in a stream of N₂. The slides were cutinto three 1″×1″ pieces and two pieces were spin coated with 1 ml of a 2mM ethanolic solution of lead (II) perchlorate. The other piece wascoated with 1 mM lead (II) perchlorate. After spin coating, the pieceswere cut in half (1″×0.5″) to provide substrates for three sandwichcells. The sandwich cells were fabricated by pairing a substrate withanother identically treated substrate. To access the effect of thicknessof the sandwich cell, one 25 micron and one 50 micron thick cell werefabricated from 2 mM coated substrate while one 25 micron cell wasfabricated from 1 mM coated substrate. Two substrates, with thefunctionalized surfaces facing each other, were separated by mylarspacers with desired thickness (e.g. 25 or 15 micron) by placing onelong mylar piece along one of the short ends and two small pieces at twocorners of the other short end. The two pieces were held together byusing binder clips. Each sandwich cell was filled with 10 μl of LC E7,by capillary action through space between the small mylar pieces.

Microfluidic cell with large single sensing area “(long sensor)”was alsoprepared using 3.8 cm×1.9 cm glass substrate coated with polymermicropillars fabricated using standard photolithography. The polymermicropillars were 5 micron tall, 10 micron diameter with 20 microncenter-to-center spacing and covered 3.5 cm×1.3 cm area on the glasssubstrate. The micropillared glass substrate was coated with with 20 Åthick titanium layer followed by 100 A thick gold film The gold coatedsubstrates were chemically functionalized by forming self-assembledmonolayer of 11-mercaptoundecanoic acid (MUA) after incubating thesubstrate in 1mM ethanolic solution for ˜16 hrs. These substrates werethen rinsed thoroughly with ethanol and dried in a stream of N₂. Thesubstrates were then briefly (15 s) subjected to UV ozone treatment,then rinsed thoroughly with ethanol and dried in a stream of N₂. Themicrostructured substrate was then spin coated with 1 mL of 2 mMethanolic solution of lead(II) perchlorate and spotted with 4 μl of a20/30.3 mixture of E7 and octane. The resultant substrate was pairedwith OTS treated substrates forming a head space above the LC film using12 micron (sensor 1), 15 micron (Sensor 2) top, 25 micron (sensor 3),and 50 micron (Sensor 4) thick mylar strips.

Microfluidic cells with 10 discrete sensing areas in 2 rows “2×5sensors” were fabricated by using a 2×5 array of micropillared area (˜5mm across) spaced ˜8.5 mm (center-to-center) apart on an ˜43 mm×17 mmglass substrates. The polymer micro pillars on the glass substrate werefabricated using standard wet photolithography and were 5 micron tall,10 micron diameter and are spaced by 20 micron (center-to-center). Themicropillared substrates were coated with 20 Å thick titanium layerfollowed by 100 A gold film The gold coated slides were chemicallyfunctionalized by forming self-assembled monolayer of11-mercaptoundecanoic acid (MUA) after incubating the substrate in 1 mMethanolic solution for ˜16 hrs. These substrates were then rinsedthoroughly with ethanol and dried in a stream of nitrogen (N₂). Theslides were then briefly (15 s) subjected to UV ozone treatment, thenrinsed thoroughly with ethanol and dried in a stream of N₂. Thesubstrate was then spin coated with 2 mM lead(II) perchlorate. Themicropillared areas (5 mm across) were then spotted two times with 0.14μl of an E7-octane mixture (25%-75%). The LC filled sensor substrate wasthen paired with another glass substrate coated with(Tridecafluoro-1,1,2,2-tetrahydrooctyl)-trichlorosilane (OTS) with thefunctionalized surfaces facing each other. Two mylar strips (12 micronthick) were placed along the long edge of the substrate to define thehead space above the LC film and the substrates were held together usingbinder clips. In a second 2×5 sensor, another mylar strip was placedalong the middle to form a “channel” separating the descrete circularareas.

The 2×5 sensor, sandwich cells, and long sensors were stored in theargon filled bag at 4° C. and exposed the following day to 1 ppm H₂S.Prior to opening the bag, all the cells were allowed to equilibrate toroom temperature for 30 minutes. After equilibration, images of 2×5sensors (FIG. 1A), long sensors (FIG. 1B), and sandwich cells (FIG. 1C)were taken before exposure to 1 ppm H₂S for eight hours (FIG. 1). TheH₂S test chamber (21 Lit volume) was equilibrated to 1 ppm H₂S at a flowrate of 4 L/min. for 15 minutes, then the sensors were placed inside,and the test chamber was equilibrated for an additional 15 minutes atthe same flow rate. The flow was then decreased to 1 L/min. and allsensors were exposed at this rate for the remainder of 8 hours.

After exposure to 1 ppm H₂S for eight hours images were acquired (FIG.2). The 2×5 sensors show responses on the first circles on both sidesand the response is not affected by the presence of an additional pieceof mylar in the center of the cell (FIG. 2A). The long sensorsdemonstrated a good response, especially with 25 micron and 50 micronmylar (FIG. 2B). Thicker mylar produced brighter reacted areas andgreater responses. The sandwich cells also gave a very small responses(FIG. 2C).

Long Sensor 3 was imaged 2 to 3 days after exposure to H₂S and the imageof the sensor was acquired. The appearance of the sensor was comparedwith that on the first day. The response of the sensor was stable (FIG.2, compare with FIG. 2B, Sensor 3).

The results from this experiment show that (i) the microfluidic cells(long sensor or 2×5 sensor) with head space are more sensitive than thetraditional sandwich cells fabricated from identically preparedsubstrate (ii) the 15 micron spacers provide for a small response andthicker spacers (e.g., 25 micron mylar) provide a greater response and(iii) relatively long cells (similar to 2×5 sensors) are appropriate forhigher concentrations of H₂S.

Example 2 Comparison Between Sensitivity of Sandwich Cells andMicrofluidic Cells

During the development of embodiments of the technology, experimentswere conducted to develop new embodiments of liquid cells to detect ananalyte (e.g., H₂S). For example, in some experiments, traditionalsandwich cells (cells with all the space between two substrate filledwith LC) were constructed and their performance to detect H₂S wascompared with microfluidic cells (cells with a head space between thetop surface and LC film) fabricated using identical protocol.

A traditional sandwich LC cell was prepared first by coating a 1″×3″aluminosilicate (AlSi) glass slide with 20 Å thick titanium layerfollowed by 100 Å thick gold film The gold coated slides were chemicallyfunctionalized by forming self-assembled monolayer of11-mercaptoundecanoic acid (MUA) after incubating the slide in 1 mMethanolic solution for ˜16 hrs. These slides were then rinsed thoroughlywith ethanol and dried in a stream of nitrogen (N₂). The slides werethen briefly subjected (15 s) to UV ozone treatment, then rinsedthoroughly with ethanol and dried in a stream of N₂. The slides were cutinto three 1″×1″ pieces and each piece was spin coated with 1 ml of a 4mM ethanolic solution of lead perchlorate. After spin coating, thepieces were cut in half (1″×0.5″) to provide substrates for six sandwichcells. The sandwich cells were fabricated by pairing these substrateswith (Tridecafluoro-1,1,2,2-tetrahydrooctyl)-trichlorosilane (OTS)treated glass substrates with similar dimensions. Two substrates, withthe functionalized surfaces facing each other, were separated by mylarspacers with desired thickness (e.g. 25 micron) by placing one longmylar piece along one of the short ends and two small pieces at twocorners of the other short end. The two pieces were held together byusing binder clips. Each sandwich cell was filled with 10 μl of LC E7,by capillary action through space between the small mylar pieces.

A microfluidic cell was prepared using 3.8 cm×1.9 cm glass substratecoated with polymer micropillars fabricated using standardphotolithography. The polymer micropillars were 5 micon tall, 10 microndiameter with 20 micron center-to-center spacing and covered 3.5 cm×1.3cm area on the glass substrate. The micropillared glass substrate wascoated with with 20 Å thick titanium layer followed by 100 Å thick goldfilm The gold coated substrates were chemically functionalized byforming self-assembled monolayer of 11-mercaptoundecanoic acid (MUA)after incubating the substrate in 1 mM ethanolic solution for ˜16 hrs.These substrates were then rinsed thoroughly with ethanol and dried in astream of N₂. The substrates were then briefly (15 s) subjected to UVozone treatment, then rinsed thoroughly with ethanol and dried in astream of N₂. The microstructured substrate was then spin coated with 1mL of 1 mL ethanolic solution of lead(II) perchlorate. The micropillaredarea of the substrate was then filled with ˜3 microliter of LC E7:octane (40:60) mixture. After evaporation of the organic solvent, theLC-filled substrate was then paired with an OTS treated glass substrateforming a head-space (e.g., a head space of approximately 20 microns),by placing two strips along the long side of the substrate, to allowcontrolled diffusion of the targeted analytes above the LC film.

Once these sandwich cells were fabricated, the LC did not exhibithomeotropic alignment on these substrates while the microfluidic cellsexhibited homeotropic alignment as expected on lead (II) perchloratetreated substrates. These results suggested that a format using amicropillared substrate may provide a better alignment of the LC on anidentically functionalized surface for analyte detection. As a result,experiments were conducted to compare performance of microfluidic cellwith headspace and sandwich cell fabricated using slightly differentpreparation protocols. The sandwich cells were fabricated as describedabove except the MUA functionalized surfaces were not treated with UVozone as described above while the microfluidic cells were fabricated asdescribed above. This process produced sandwich cells with good initialLC alignment.

Experiments were performed by manufacturing six sandwich cells andexposing them alongside six microfluidic cells (e.g., prepared asdescribed above) to compare responses. The sensors were manufactured andstored at 4° C. until the experiment was performed. Two microfluidiccells and two sandwich cells were assigned to be exposed to air at 45%RH (negative controls) and an additional four microfluidic cells andfour sandwich cells were assigned to be exposed to 8 ppm H₂S at 45% RHfor eight hours.

On the day of the exposure, microfluidic cells and sandwich cells wereallowed to equilibrate at room temperature for 15 minutes before openingthe bags. Images were taken before the microfluidic and sandwich cellswere exposed to H₂S and of sensors assigned as negative (i.e. exposed to45% zero air) controls (FIG. 4 and FIG. 5). Before exposure, thesandwich cells appeared homeotropic with the exception of one brightspot in one of the cells assigned as negative control cells (FIG. 4) andone of the microfluidic cells (FIG. 5). They were then placed in a large(˜21 L) test chamber and the H₂S flow inside the chamber was maintainedat 5.8 liter/min for initial 15 minutes. After the cells had been in thetest chamber for 15 minutes at a 5.8 liter/minute flow rate, the flowrate was decreased to 1.2 liter/minute for the remainder of theeight-hour exposure.

All of the microfluidic cells and sandwich cells were exposed togetherto either H₂S or zero air (FIG. 6 and FIG. 7). There are small responsesthat can be seen along the edge of the sandwich cells. The responses arebarely visible after the eight hour exposure (FIG. 6A). The sandwichcells exposed to zero air show no responses after the eight hourexposure (FIG. 6B). The microfluidic cells show responses ofbirefringent fronts ranging from 17.4 to 20.4 mm (FIG. 7A). Themicrofluidic cells that were exposed to zero air show no responses afterthe eight hour exposure (FIG. 7B). Neither the microfluidic nor thesandwich cells showed any response to zero air at 45% RH.

These results establish that the microfluidic cells (i) provide morestable alignment of LC on similarly functionalized surface and (ii) aremore sensitive to the target analyte (H₂S) compared to traditionalsandwich cells.

Example 3 Microfluidic Cell as a Cumulative Dosimeter

During the development of embodiments of the technology provided herein,experiments were conducted to establish that the microfluidic cellscould be used as cumulative dosimeter by determining the relationshipbetween response from these microfluidic cells “(cumulative sensors)”and analyte dose. Experiments were conducted using a cumulative analytesensor in an average environmental condition of 22° C. and 45% relativehumidity (RH).

Cumulative Sensor Fabrication

Cumulative sensors were designed to measure a total dose of gaseous H₂Sin a sample by integrating over the concentration and exposure time.Cumulative sensors were prepared using 3.8 cm×1.9 cm glass substratescoated with polymer micropillars fabricated using standardphotolithography. The polymer micropillars were 5 micon tall, 10 microndiameter with 20 micron center-to-center spacing and covered 3.5 cm×1.3cm area on the glass substrate. The micropillared glass substrate wascoated with with a 20 Å thick titanium layer followed by a 100 Å thickgold film. The gold coated substrates were chemically functionalized byforming a self-assembled monolayer of 11-mercaptoundecanoic acid (MUA)after incubating the substrate in 1 mM ethanolic solution for ˜16 hrs.These substrates were then rinsed thoroughly with ethanol and dried in astream of N₂. The substrates were then briefly (15 s) subjected to UVozone treatment, then rinsed thoroughly with ethanol and dried in astream of N₂. The microstructured substrate was then spin coated with 1mL of ethanolic solution of lead(II) perchlorate. The micropillared areaof the substrate was then filled with ˜3 microliter of LC E7:octanemixture at 40:60 ratio. After evaporation of the organic solvent, theLC-filled substrate was then paired with an OTS treated glass substrateforming a head-space (e.g., a head space of approximately 20 microns) byplacing a U-shaped mylar strip around edge of the substrate, to allowcontrolled diffusion of the targeted analytes above the LC film. Thesubstrates were held together using binder clips and three sides withthe mylar strips were sealed using low off-gassing epoxy to force thegas diffusion through the open end of the sensor. These cumulativesensors, fabricated in a lot of five, were stored in an argonenvironment prior to use.

Cumulative sensors were exposed inside a large (21 liter) exposurechamber to 8 combinations of H₂S concentration and time at 22° C. and45% RH to determine the relationship between sensor response and H₂Sdose (concentration x time). Sensors were also exposed to H₂S free airat 22° C. and 45% RH as a control. Four sensors were tested per dose. Anin-house gas delivery and testing apparatus that is capable to generateH₂S concentrations from low ppb to 100 ppm was used to test the sensors.The exposure system also allows generation of desired concentrations atdesired temperatures and humidity. For each test point 5 sensors weremade as described above; 1 sensor was exposed to 16 ppm-hours at roomtemperature and 45% RH as a reference exposure for quality control andthe other 4 sensors were exposed to the test dose.

Additional experiments tested a microfluidic cell for measuring H₂S thatwas prepared as described in Example 1.

Quality Verification and Test Procedure

On the day of exposure, the package of five sensors was first allowed toequilibrate at room temperature for 15 minutes. After equilibration, onesensor was used as a QC sensor. For each lot, a QC sensor was exposed to8 ppm H₂S at 45% RH for 2 hours (nominal dose of 16 ppm-hr). The H₂Stest chamber (21 liter volume) was equilibrated at a flow rate of 5.8L/minutes for 15 minutes before exposure; the sensor was imaged, andthen placed inside the test chamber. The test chambers were equilibratedfor an additional 15 minutes at the 5.8 liter/minute flow rate. The flowwas then decreased to 1.2 liter/minute and the sensor was exposed atthis rate for the remainder of the 2 hours and 5 minutes. The sensor wasagain imaged after exposure. Then the sensor was left at ambienttemperature for 6 hours and imaged again to check for any change inresponse.

In parallel, the remaining four sensors were exposed to the desired testconcentrations for specified time inside another identical exposurechamber following similar procedure.

Measurement of Response

After exposure to the QC dose or the desired dose, digital images of thesensors were acquired using a digital scanner interfaced with a laptop.During the development of embodiments of the technology provided herein,several different methods were evaluated for determining sensor responseto analyte by analyzing images of exposed sensors. While the technologyis not limited in the methods that are appropriate for quantification ofimages, ImageJ (NIH) was used in some trials for analyzing sensor imagesacquired from a scanner. Using the ImageJ freeware, a simple method tocalculate the width of the bright front (i.e. the distance the brightfront has travelled in an otherwise dark background) was developed.Using this method, the response length was measured for both the QCsensor and the sensors exposed to different doses.

Results

Images acquired of sensors exposed to 0, 0.8, 4, 8, 25, 40, 80, 120, and160 ppm-hours of H₂S are provided in FIG. 8A through 8I. FIG. 8J showsan image acquired for the quality control experiment in which a sensorwas exposed to 16 ppm-hours of H₂S.

The average widths of the response fronts were determined using ImageJand are plotted in FIG. 9. The consistent curve demonstrates that thesensors do provide a response to total H₂S dose and are not skewed byshifting time and concentration combinations. The relationship betweenresponse length and dose resembles a square root curve. Plottingresponse length versus the square root of dose produces a linearrelationship (FIG. 10). Without being bound by theory, the square rootrelationship most likely results from this being a process governed bylaws of diffusion of gas.

After the dose response curve was generated, a verification test wasperformed by making four lots of sensors following the same protocol andexposing them to different concentrations. FIG. 10 shows that responsefrom the 3 verification runs (denoted by triangles) were on the sameline and performed very closely to expected performance.

By fitting a plot of the square root of the dose versus response length,an equation was derived to correlate the measured dose from the responselength of a sensor:

Furthermore, quality control criteria were developed during thedevelopment of embodiments of the technology. The sensors exposed toestablish QC criteria were reasonably consistent and a QC test windowwas established.

Based on experiments in which 11 exposures at 8 ppm for 2 hours (16ppm-hour) were recorded, an average response of 8.11 mm was measured.The sensors from the QC test were left at ambient (room) temperature for6 hours after their H₂S exposure and then imaged again. The amount ofresponse changed very little over the 6 hours indicating good responsestability. These results indicate that (i) the microfluidic cells can beused as cumulative dosimeters and (ii) the total exposure dose can becalculated by measuring the length of the change in LC orientations.Using the algorithm one can use these dosimeters to determine an unknownconcentration.

In addition, a microfluidic cell for measuring H₂S was prepared andtested as a sensor for environmental monitoring. In these experiments,embodiments were tested that were designed to detect lowerconcentrations of H₂S over longer time periods of time, which isappropriate for environmental monitoring. In particular, themicrofluidic cell was tested under conditions appropriate for a devicedesigned to detect approximately ppm-level concentrations of H₂S duringa typical work shift (e.g., for 8 to 12 hour exposures).

Sensors were exposed to H25 as described above and the minimum timesneeded for a response was recorded. In these experiments, a response isdefined as the first visually detectable bright front observed on thesensor (typically ˜0.5 mm in length). Data were collected for exposureto H₂S over a range of 15 to 50 ppb. Responses were observed for H₂Sconcentrations of 15 ppb, 16.67 ppb, 25 ppb, and 50 ppb at exposuretimes of 72 hours, 72 hours, 48 hours, and 24 hours, respectively.

These results indicate that much lower concentrations of target gasesare detectable by increasing the exposure time. Such performance is ofvalue for environmental monitoring where lower concentrations of toxicgas may be present for extended times.

Example 4 Effect of Head Space Height of the Microfluidic Cells onSensitivity

During the development of embodiments of the present technology,experiments were performed to determine the effect of head space heighton the sensitivity of the microfluidic cells (“cumulative sensors”).

Cumulative sensors were prepared using 3.8 cm×1.9 cm glass substratescoated with polymer micropillars fabricated using standardphotolithography. The polymer micropillars were 5 micron tall, 10 microndiameter with 20 micron center to center spacing and covered 3.5 cm×1.3cm area on the glass substrate. The micropillared glass substrate wascoated with a 20 Å thick titanium layer followed by a 100 Å thick goldfilm The gold coated substrates were chemically functionalized byforming self-assembled monolayers of 11-mercaptoundecanoic acid (MUA)after incubating the substrate in 1 mM ethanolic solution for ˜16 hrs.These substrates were then rinsed thoroughly with ethanol and dried in astream of N₂. The substrates were then subjected briefly (15 s) to UVozone treatment, then rinsed thoroughly with ethanol and dried in astream of N₂. The microstructured substrate was then spin coated with 1mL of 1 mM ethanolic solution of lead(II) perchlorate. The micropillaredarea of the substrate was then filled with ˜3 microliter of LC E7:octane(40:60) mixture . A U-shaped polymer strip (˜1 mm wide) was patterned onanother glass substrate using photolithography. The thickness of thepolymer strip was maintained at 25 micron. This substrate was coatedwith OTS and paired with the LC filled substrate to form a head-space 20microns thick head space. A U-shaped mylar film was placed over thepolymer strip to generate microfluidic cell with 45 micron head space.The substrates were held together using binder clips and three sideswith the polymer/mylar strips were sealed using low off-gassing epoxy toforce the gas diffusion through the open end of the sensor. Thesecumulative sensors, were stored in an argon environment prior to use.

The cumulative sensors were then exposed to 1 ppm H₂S at 45% RH inside asmaller exposure chamber (9.5×7.5×4.5 cm³) at a flow rate of 5 L /min.The exposure chamber was placed between two crossed polarizers and wasflanked by a CCD camera and diffused light source. The digital images ofthe cumulative sensors were captured in real-time as the sensor wasexposed to the H₂S gas. The captured images were then analyzed todetermine the length of the bright front as a function of exposure timeusing ImageJ (NIH). FIG. 11 shows the variation of the response lengthas a function of the square root of the exposure time. The results showthat the response length, after an initial delay, increases linearlywith square root of the exposure time. And also shows that thecumulative sensors with thicker head space height are more sensitive(shorter delay and higher response length) than the sensors with thinnerhead space height. These results again establish that the microfluidiccells can be used for tuning the dynamic range of detection of differentgases.

Example 5 Microfluidic Cells as Real-Time Sensors

During the development of embodiments of the present technology,experiments were performed to demonstrate that the microfluidic sensorscan be used as real time sensors. The microfluidic cells were fabricatedas described in Example 4. These cells were then exposed to differentconcentrations of H₂S at 45% RH inside the small exposure chamber at 500ml/min flow rate. The images of the sensors were then captured inrealtime using CCD camera. These images were then analysed to determinethe response length (the width of the bright front). FIG. 12 shows thevariation of the response length as a function of the square root of theresponse time. The results show that the response length, after aninitial delay, shows linear behavior with the square root of theexposure time. The slopes of these straight lines are different fordifferent concentrations. These results indicate that measurement of theslope of these lines in real-time can provide a basis for a real-timesensor using these microfluidic cells.

During the development of embodiments of the present technology,experiments were performed to demonstrate that some modifications in themicrofluidic cell format can lead to detection of analyte withsignificantly shorter initial delay. Microfluidic cells were fabricatedusing the rectangular micropillared area as described in Example 4except after filling the micropillared area with LC, the excess glass onthe short side of the substrate extending to the edge of the substratewas removed so that micropillared area extended to the edge of thesubstrate. This substrate was then paired with another glass piececoated with OTS to form a ˜45 micron head space between the top surfaceand LC surface. Two substrates were then glued together along threesides using low-off-gassing epoxy. The microfluidic cell was thenexposed inside the small exposure chamber (9.5×7.5×4.5 cm³) at a flowrate of 5 L/min while its appearance was captured in real-time using aCCD camera. The digital images were then analyzed to determine theresponse length as a function of exposure time. FIG. 13 shows thevariation of the response length as a function of time. The resultsclearly show that with this cell configuration detection of 1 ppm of H₂Sis possible within 4 minutes of delay.

During the development of embodiments of the present technology,experiments were performed to determine if the microfluidic cells canactually be used to detect the change in the concentration of H₂S.

A microfluidic cell with 20 micron head space was fabricated asdescribed in Example 4 and sequentially exposed to differentconcentrations 1, 2, 5, 1, and 2 ppm H₂S for 2, 1, 1, 1, 1, hoursrespectively. The H₂S concentrations were maintained at 45% RH at a flowrate of 500 ml/min inside a small (9.5×7.5×4.5 cm³) exposure chamber.The response of the microfluidic cell to the exposure was recorded inreal-time and the images were analyzed. FIG. 14 shows variation of theresponse length as a function of the exposure time. The results showthat the microfluidic cell actually responds to the change inconcentration of H₂S whether it is increasing or decreasing. As shown inthe insets of FIG. 14, the change in concentration can be detectedwithin few minutes of the change in concentrations. These results whencombined together demonstrate that the microfluidic cells designed forcumulative measurement of gas analyte can be used for real-timedetection.

Example 6 Microfluidic Cells for Cumulative Detection of Formaldehyde

During the development of embodiments of the technology provided,experiments were performed to demonstrate that the microfluidic cellscan be used to detect HCHO (formaldehyde) using LC-based detectiontechnology. In this regard, first detection of HCHO was demonstratedusing simple LC sensors. Next, microfluidic cells were fabricated usingthe LC and the micropillared substrate to demonstrate that these cellscan be used for cumulative detection of HCHO.

HCHO sensors were fabricated on patterned glass substrates decoratedwith polymer micro-pillars using conventional lithographic techniques.The array of micropillars covered a 5 mm diameter area of the substrate.A droplet of LC (methoxybenzilidene butylanaline i.e. MBBA), whendeposited onto the array of micro-pillars was subjected to capillaryforces that caused the LC to spread to form a stable film. The pillarheight (5 microns) determined the thickness of the film These sensorswere then exposed to vapors generated by bubbling nitrogen (N₂) throughthe targeted chemicals. For example, HCHO vapor was generated bybubbling N₂ through HCHO in deionized water. The desired concentrationsof HCHO were generated by diluting the saturated stream with dry N₂. TheHCHO concentration was determined using a commercial HCHO detector(FP-30, RKI Instruments Inc.).

The film of LC, when supported on the sensor surface, possesses opticalbirefringence and leads to a bright optical appearance of the sensor.Upon exposure to 17.5 parts per million (ppm) HCHO, the LC changes to anisotropic liquid with no optical birefringence and the sensor changes toa dark appearance between crossed polarizers. A measurement of theamount of light transmitted through the sensor as a function of timeprovides the dynamic response of the sensor to HCHO. The sensor yields ameasurable response to 17.5 ppm HCHO (with 50% RH) in approximately 5minutes (FIG. 15).

Furthermore, the data demonstrate the high selectivity of this sensorfor HCHO relative to vapors from other chemicals representative ofalcohol and ketone groups. Using this sensor system, 7 ppm HCHO wasdetected in less than 30 minutes. In addition, experiments measured thethreshold concentrations of toluene, hexane, benzene, and isopropanolvapors required to induce a phase transition in a range of LCs (e.g.,MBBA, SCB, E7, TL205, MLC-7800) to be greater than 1000 ppm. Withoutbeing bound by theory, it is believed that the response of MBBA to lowppm concentrations of HCHO reflects, specific interactions between HCHOand the 4-butylaniline, one of the components present in MBBA. However,an understanding of the mechanism is not required to practice thetechnology.

Experiments were performed to collect data demonstrating use of theLC-based principles for cumulative detection of HCHO. Embodiments of thetechnology were fabricated (e.g., in the form of a cumulative dosimeter)and exposed to 7 ppm HCHO. The dosimeter was fabricated by using a 3.8cm×1.9 cm glass substrate with a 3.5 cm×1.3 cm micropillared area. Themicropillared area was filled with the LC using capillary action to forma thin (5 micron) film. This sensor substrate was then paired with aclean glass substrate with a head-space (e.g., a head space ofapproximately 45 microns) to allow controlled diffusion of the targetedanalytes above the LC film (FIG. 16a ). When viewed betweencrossed-polarizers, the dosimeter initially appeared bright (FIG. 16b ).The dosimeter was then exposed to 7 ppm HCHO at a flow rate of 200ml/minute for 8 hours. Dark fronts appear in ˜30 minutes and progressinward with time (see, e.g., FIG. 16b ). As a result, the measured lightintensity decreases linearly with exposure time (FIG. 16c ). The resultsdemonstrate that exposure to HCHO leads to a phase transition of the LCthat is evidenced as a dark front on each side of the dosimeter and thatevolves as a linear function of exposure time. The dark front moveslinearly with cumulative exposure to HCHO (FIG. 16c ). Additionally, thefronts stay unchanged for days in an ambient environment, suggestingthat these dosimeters provide a stable reading of cumulative exposureover a typical (e.g., 8-hour) work shift. Accordingly, these datademonstrated the feasibility LC-based microfluidic cell to create apassive dosimeter badge for toxic gases. The LC-based dosimeters aresmall (˜4 cm×2 cm), light weight (<5 g), and easily read by lightintensity measurement to indicate cumulative exposure to an analyte inthe gas phase, e.g., HCHO.

Example 7 Channel-Based Analyte Sensor

During the development of embodiments of the present technology,experiments were performed to test devices comprising a channel (e.g., amicrofluidic channel) for use in methods in which a functionalizedsurface in the channel is first exposed to an analyte and then thereacted functionalized surface is exposed to a liquid crystal forreading the device. While the embodiments tested demonstrate the generalapplicability of the technology, data were collected for an exemplarydevice comprising a gold surface functionalized with a self-assembledmonolayer (SAM) of 4-aminothiophenol (ATP) to detect oxides of nitrogen(e.g., NO or NO₂, e.g., via conversion of NO to NO₂). Amethoxybenzilidene butylanaline (MBBA) liquid crystal was applied to thereacted ATP surface to measure the NO₂ levels.

Some applications of gas monitoring relate to detecting low levels(e.g., parts per billion) of gases. For example, some biomedicalapplications relate to detecting nitric oxide (NO) gas in human breathfor asthma monitoring. Detection of NO via conversion to NO₂ anddetecting the NO₂ using various compounds (e.g., substitutedaniline-based compounds) has been identified as one approach fordetecting NO. Among these approaches, ATP was identified as asubstituted aniline compound for use in a NO detector. Experiments wereconducted using a Fourier transform infrared spectroscopy (FTIR)instrument equipped with a Specular Apertured Grazing Angle (SAGA)accessory to detect functional groups present on a SAM surface. Inparticular, experiments were performed to re-examine the ATP SAM, LCalignment on an ATP SAM coated surface, and detection of NO₂ using thesesurfaces.

Characterization with FTIR SAGA

Glass slides coated with 1000 A gold were immersed in ˜1 mM ATP for ˜16hours. The substrates were rinsed with ethanol and dried in a stream ofN₂. Using FTIR-SAGA, a background FTIR signal was acquired with a baregold substrate that was soaked in ethanol overnight. The FTIR spectrumof ATP SAM was collected using the background spectrum as a reference.The ATP peaks were clearly visible with three prominent peaks at 1480,1590, and 1620 cm⁻¹, corresponding to v_(ee), v_(ee), and δ_(NH) modes,respectively. In some experiments, inverted peaks in the hydrocarbonregion (2900 cm⁻¹) indicated contamination of the reference gold surfacethat was resolved by cleaning the surface with a butane torch. Once theFTIR spectrum was collected, the substrate was placed inside an exposurechamber and exposed to 3.5 ppm humid NO₂ (50 sccm 4.7 ppm NO₂ and 20sccm N₂ bubbled through H₂O) for approximately 30 minutes. The FTIRspectrum of the exposed surface showed some change in these three peaks,although no new prominent peaks were observed (FIG. 17).

Next, to test the ability of the instruments to detect the presence ofATP film on a 100 Å thick gold film, experiments were performed withsilicon wafers. A fresh (e.g., one-day old) gold coated silicon waferwas incubated in 1 mM ATP for ˜16 hrs, rinsed with ethanol, and dried ina stream of N₂. Background spectra were collected from a piece of thesame silicon substrate that was rinsed with ethanol and dried in streamof N₂. The peaks were well resolved and with the presence of somebackground moisture (FIG. 18).

Experiments were performed to test the ability of SAGA to resolve theFTIR spectra on 100 Å gold deposited on a glass substrate. For theseexperiments, two fresh gold (100 Å) coated microscope slides wereincubated in 1 mM ATP for ˜16 hours, rinsed with ethanol, and dried in astream of N₂. A background spectrum was collected from an identicallyprepared slide immersed in ethanol alone. The microscope slide wasbroken into two pieces. One half of the slide was used for FTIRexperiments and the other half was used to prepare a LC cell. Analysisof the first halves of the slides showed that the FTIR peaks were wellresolved (FIG. 19). The SAM surface was also exposed to 3.25 ppm NO₂(100 sccm 4.2 ppm mixed with 20 sccm N₂ bubbled through water) forapproximately 30 minutes. The film exposed to NO₂ showed a decrease inpeak intensity and a small shift in the peak positions (FIG. 19). Theseexperiments demonstrated that changes are detectable in the ATPmonolayer on 100 Å thick gold surface upon exposure to NO₂ (e.g., byusing FTIR).

Alignment of Different LCs on ATP Treated Surface:

After confirming the presence of an ATP monolayer using FTIR (above),experiments were conducted to prepare an ATP functionalized surface andevaluate the alignment of liquid crystals on the surface.

To prepare ATP-functionalized surfaces, glass slides were cleaned byultrasonicating for 10 minutes in detergent, then rinsed in deionizedwater and washed in acetone and ethanol (reagent grade). A ˜2 mM ATPsolution in ethanol (˜68.4 mg ATP dissolved in 200 ml ethanol) wasprepared. Fresh gold coated slides were incubated on clean stainingdishes; one staining dish was filled with ethanol for reference and twowere filled with the ATP solution. Slides were incubated overnight,rinsed twice in 100% ethanol, and dried in N₂ before use.

To evaluate the alignment of liquid crystals on ATP treated surfaces,seven different liquid crystals having varying physical properties weretested:

A 5CB

B E7

C TL-205

D MLC-7800

E MLC-6488

F MBBA (Δε<0)

G MLC-14200 (high Δε)

A series of 1″×3″ substrates were tested under the following conditions:

1 incubated in ethanol

2 functionalized with ATP

3 functionalized with ATP and exposed to NO₂

4 functionalized with ATP and exposed to N₂.

The substrates were cut into five pieces and paired with OTS treatedslides and lifter slips before filling the devices with a LC in nematicphase. The alignment of LCs on these films was monitored for at leastsix days.

The results from these experiments show that 5CB aligns homeotropicallyon a bare gold substrate that was incubated in ethanol overnight (FIG.20A). In addition, MBBA assumes a homeotropic alignment on an ATPfunctionalized surface and adopts a planar alignment on NO₂-exposed ATPfunctionalized surface (FIG. 20B). In these experiments, the other LCstested did not show any significant change in LC alignment upon exposureto NO₂ (FIGS. 20A and 20B).

To ensure that the change in LC alignment upon exposure to humid NO₂ wasnot from humidity alone, surfaces were prepared and exposed to 3.5 ppmNO₂ at 70 sccm (50 sccm 4.7 ppm NO₂ and 20 ppm N₂ bubbled throughwater). Identical surfaces were also prepared by exposing them to humidN₂. The results showed that the (i) ATP functionalized surface can beused to achieve homeotrpic alignment of LC MBBA and (ii) LC MBBA can beused to report interaction of NO₂ with the ATP surface (FIG. 21).

Characterization of ATP Treated Surface:

The previous experiments suggested that a change occurred at the ATPsurface as a result of exposure to NO₂ that can be reported by the LCMBBA. During the development of embodiments of the technology,experiments were conducted to characterize the surfaces using FTIR,contact angle, and ellipsometric measurements. These experimentsestablished the reproducibility of the functionalized surfaces preparedas described herein on 1000 Å gold and 100 Å gold surfaces. For theseexperiments, the SAM coated slides were first prepared as describedherein and then broken into two halves. One half was used for FTIRexperiments and the second half was used for contact angle andellipsometric measurements for the 1000 Å thick film

FTIR spectra acquired of the ATP SAM were comparable to grazing angle IRspectra reported in the literature (see, e.g., Langmuir 12 15 (1996)3689). The spectra show a decrease, especially at ˜1480 cm⁻¹, in thepeak intensity upon exposure to NO₂ (FIG. 22). FTIR spectra of ATPfunctionalized surfaces and surfaces exposed to humid NO₂ and humid N₂are shown in FIG. 22. These results indicate that there are changes inthe ATP surface upon exposure to NO₂ and the changes are not due to thepresence of humidity.

Detection Using Channels:

The results of experiments performed during the development ofembodiments of the technology described demonstrated that using asurface functionalized with ATP detects the presence of NO₂ by using LCMBBA. Experiments were conducted to verify that passing NO₂ gas througha narrow channel allows a faster reaction and therefore a sensitivedetection with shorter response time (e.g. 30 min).

During the development of embodiments of the technology, apolydimethylsiloxane (PDMS) channel was prepared to provide a welldefined gas flow path on an ATP treated substrate. A “master” wasprepared by gluing a ˜1 mm wide, 3 inch long, and 0.7 mm thick glasspiece on a 1 inch×3 inch glass substrate using UV curable epoxy. A filmof PDMS mixture (prepolymer and curing agent) was overlaid on the masterand cured at 60° C. for 1 hour. The PDMS channel was removed frommaster, overlaid on the ATP functionalized surface and NO₂ gas waspassed through the channel for 10 minutes using a small needle at theend of the gas delivery system. The exposed surfaces were then used toassemble LC cells by pairing them with OTS coated slides using 25 micronmylar as spacers. The LC cell was filled with MBBA in isotropic phase at50° C. NO₂ concentrations of 88 ppb and 50 ppb were generated by mixing4.7 ppm NO₂ from a certified cylinder (4.7 ppm) and humid nitrogengenerated by bubbling nitrogen through DI water at appropriate ratios.For negative controls, N₂ was bubbled through deionized water.

The results show detection of 50 ppb NO₂ by the ATP coated surfaceexposed along the path defined by PDMS channel (FIG. 23). The resultsindicate that measuring the length of the bright path along the channelprovides a means to quantify the response of the chemicallyfunctionalized surface to an analyte.

These results suggested that a longer channel having controlleddimensions would provide a more precise and accurate quantification ofsensor response. Experiments were performed with PDMS channels preparedusing an aluminum master having a 1 mm wide×1 mm thick ridge definingchannel dimensions. To minimize the potential variation duringintroduction of the gas, a small needle (˜1 mm diameter) was integratedinto the PDMS block with channels (see FIG. 24). The ATP treatedsubstrates were then consistently placed over the PDMS channels andexposed to different concentration of NO₂. Before exposure, thesubstrate was firmly pressed against the PDMS block and a weight wasplaced over the substrate during the exposure. After exposure, the PDMSchannel was removed and the exposed surface was paired with an OTStreated glass substrate forming a 25 micron cavity using a mylar spacer.The two substrates were aligned leaving a small lip along the long edgeof the cell to facilitate LC filling. The cell was secured and placedinside an oven at 60° C. LC MBBA was applied along the lip and allowedto spread inside the cell for 5 minutes while the cell remained insidethe oven. After 5 minutes, the cell was taken out of the oven and the LCcell was imaged at different time intervals starting from 2 minutes.

Exposure experiments were performed with different concentrations anddifferent equilibration times. The results show that 25 ppb NO₂ isdetected (FIG. 25).

A number of experiments were performed at different concentrationsranging from 10 ppb to 80 ppb with a 10 ppb initial and then 20 ppbincrements (FIG. 26). The images from three different replicates werecaptured and analyzed 4 minutes after removing the cells from the oven.These results show that this method detects 20 ppb of humid NO₂ after 10minutes exposure with a resolution of 10 ppb. The results also show thatmeasurement of the length of the reacted channel provides a facilemethod to quantify the response as a function of concentration. Theresults also show that there is a linear relationship between the lengthof the bright channel and the concentration (FIG. 26B,).

Effect of Liquid Crystal Composition

During the development of embodiments of the technology, liquid crystalcompositions other than MBBA were tested. MBBA has negative dielectricanisotropy (Δε<0). Since most of the materials tested previously had apositive dielectric anisotropy (Δε>0), it was anticipated that LCs withnegative dielectric anisotropy might align homoeotropically on ATPfunctionalized surfaces.

LC MLC-2080 and MLC-2081 (Merck), both with negative dielectricanisotropy, were tested. Both of these LCs have a nematic-transitiontemperature above 80° C. ATP treated surfaces were prepared using theprotocol developed herein. The substrates were broken into six piecesand paired with OTS coated lifter slides. Two cells were then filledwith MBBA, MLC-2080, and MLC-2081 in nematic phase. Images were takenafter 15 minutes to assess the alignment of these LCs on the ATP coatedsurfaces (FIG. 27). The results suggest that MBBA aligns on the ATPcoated surface (FIG. 27A) and that neither MLC-2080 nor MLC-2018 alignshomeotropically on the ATP coated surface (FIGS. 27B and 27C,respectively). The homeotropic alignment does not correlate with thedielectric anisotropy and it is contemplated that the homoeotropicalignment of MBBA on an ATP treated surface is due to the functionalgroup present on MBBA.

To determine the effect of mixing MBBA with another LC having a negativedielectric anisotropy, MBBA was mixed with MLC-2080 at different volumeratios (pure MLC, 0.9 MLC, 0.8 MLC, 0.7 MLC, 0.6 MLC, 0.5 MLC, 0.25 MLC,and pure MBBA; see FIG. 28(a) through (h), respectively). The mixtureswere vortexed, heated to 65° C. for 5 minutes and vortexed again. Themixtures were monitored over several days to ensure that no phaseseparation occurred over time.

Two identically prepared ATP functionalized surfaces were broken into 8different pieces. Each piece was then paired with a piece from an OTScoated glass substrate, thus forming a 25 micron thick LC cell. One celleach from each slide was then filled with the LC mixture at 60° C.inside an oven after equilibration for 2 minutes. After 2 minutes ofequilibration, each cell was filled serially inside the oven. After 5minutes, the oven was closed and the cells were equilibrated for another5 minutes. The LC cells were imaged in the order they were filled.Images were taken after another 5 minutes of annealing at 60° C. Theresults suggest that the mixture of MLC-2080 and MBBA alignshomeotropically on an ATP treated surface at a concentration as high as80% (FIG. 28). The incubation time inside the 60° C. oven needed toachieve homeotropic alignment increases with increase in MLCconcentration. These results indicate that mixtures of LCs provide, insome embodiments, better sensitivity to NO₂ than a single LC (e.g.,MBBA) alone.

To confirm this result, identically prepared ATP coated surfaces wereexposed using the PDMS channel to 20 ppb NO₂ for 5 and 10 minutes, andthey were then filled with pure MBBA or a mixture of MBBA and MLC-2080at a 40:60 v/v ratio. Optical images were acquired of the cellsfabricated using surfaces exposed to 20 ppb NO₂ and filled withdifferent LC mixtures (FIG. 29). The images were taken after 10 minutesof equilibration at ambient temperature after removal from a 65° C.degree oven. The results show that 20 ppb NO₂ is detected using pureMBBA after a 10 minutes exposure (FIG. 29(a)). The surface exposed to 20ppb NO₂ for 5 minutes and using pure MBBA does not give any measureableresponse (FIG. 29(b)). However, the mixture of MBBA and MLC-2080 at a40:60 v/v ratio provides a measurable response after 5 minutes ofexposure (FIG. 29(c)). The data show that (i) a longer exposure timegives more time for NO₂ to react and therefore provides a greatermeasurable response and (ii) sensitivity of detection can be improved byusing MBBA-MLC2080 mixtures.

Effect of Flow Rate

Experiments described above demonstrate that using the mixture MBBA andMLC-2080 at a 40:60 v/v ratio detects the presence of 20 ppb NO₂ after a5 minute exposure at 100 sccm. Next, experiments were performed todetermine if increasing the flow rate improves the sensitivity ofdetection. To test the flow effect, identical surfaces functionalizedwith ATP were prepared and exposed to different concentrations of NO₂ atdifferent flow rates. The LC cells were fabricated and filled with pureMBBA and MBBA and MLC-2080 at a 40:60 v/v ratio. The images of the LCcells were collected 4 minutes after the cells were taken out of theoven (FIG. 30).

The results show that pure MBBA is anchored strongly so that at 800sccm, NO₂ could not induce the planar alignment along the channel. The800 sccm flow rate is too high and could potentially induce leaksoutside the channel so that the mixture adopts a planar alignment with 2minutes of exposure to 10 ppb or 30 seconds of exposure to 20 ppb. Thecells detect 10 ppb NO₂ after two minutes of exposure at a flow of 400sccm. It is contemplated that cells that are resistant to leaking underhigh flows detect the presence of 20 ppb after 30 seconds of exposure.

Channel-Based Analyte Sensor with Higher Resolution

During the development of embodiments of the technology provided herein,a dose response curve for an analyte gas was developed for a cellcomprising a gold surface and 4-aminothiophenol. The cells were thenapplied to detect low concentrations of the analyte gas.

Test conditions were standard (see above) except that all NO₂concentrations were equilibrated overnight and a 35% MBBA / 65% MLC-2080liquid crystal mixture was used. Concentrations of NO₂ tested were from10 to 40 ppb in 5 ppb increments and 40 to 100 ppb in 10 ppb increments.Controls were 0 ppb NO₂ and pure nitrogen. The LC cells were exposed todifferent concentrations of NO₂, imaged (FIG. 31), and analyzed bymeasuring the total distance of continuous LC disruption. Themeasurement of the length of LC disruption was performed using software,e.g., by overlaying the image with a technical drawing of the channelmarked with hash marks in millimeter increments. The extent ofcontinuous LC disruption in the cells was matched with the drawing todetermine the length of the phase changed path.

The results show that low amounts of NO₂ (e.g., ppb NO₂) are detected. Alinear relationship exists between the NO₂ concentration and the lengthof the bright channel on the surface functionalized with ATP (FIG. 32).

Conclusions

In sum, these experiments demonstrate that ATP functionalized surfacespromote a homeotropic alignment of MBBA and a MBBA/MLC-2080 mixture ofup to 80% of the MLC component. The time it takes for homeotropicalignment increases with an increase in the MLC concentration anddecreases with an increase in incubation temperature. The homeotropicalignment can be obtained with an ATP surface alone without a need for atop OTS treated surface. The ATP functionalized surfaces, when exposedto NO₂, promote planar alignment. The change in the ATP surface wasverified by FTIR in the form of a decrease in the peak intensitiescorresponding to the C—C and N—H stretches. A micro fluidic channelprovides a method to quantify the response to NO₂ by measuring thelength of the reacted path of the channel as reported in the form of theplanar alignment of the LC. A higher flow rate combined with theMLC-MBBA mixture detected 10 ppb NO₂ at high humidity with 2 minutes ofexposure.

Example 8 Identification of Surface Composition for Organic VaporDetection Using Liquid Crystals

During the development of embodiments of the technology provided herein,experiments were conducted to identify surface combinations fordetecting VOC (e.g., toluene vapor) with a liquid crystal (LC). Inparticular, data were collected from testing combinations of a substrateand a liquid crystal suitable for determining an unknown concentrationof toluene in the vapor phase. Several physical characteristics of theLC were monitored as indicators of VOC concentration, including LC phasetransition, LC orientation change induced by change in the structure ofpolymer film due to swelling, and dissolution of toluene-solublematerials into the LC to initiate LC orientation change. Althoughparticular experiments described herein are focused on toluenedetection, similar embodiments comprising combinations of substrate andLC are appropriate for detecting other volatile organic compounds.

The experiments used LC materials that comprised rod-shaped organicmolecules such as cyano-biphenyls (e.g., 5CB (4-cyano-4′-pentylbiphenyl)and E7 (a liquid crystal mixture comprising several cyanobiphenyls withlong aliphatic tails). These molecules form condensed phases thatpossess crystal-like long range orientational ordering but lackpositional ordering. The long-range ordering of molecules within the LCgives rise to anisotropic optical properties that result in a bright ordark appearance of the LC when viewed between crossed polarizers with abacklight source. Experiments were performed to confirm that toluenevapor causes a change in the polymer surface supporting a thin film ofLC, e.g., due to polymer swelling or due to dissolution of the polymersurface material into the LC exposed to toluene. Both these phenomenainduce a change in the LC orientation on the surface that can be easilydetected by monitoring a change in incident light intensity. Forexample, absorption of toluene directly into the LC phase disrupts thelong-range order of the LC, thus giving rise to a phase transition to anisotropic material and producing distinct changes in the opticalappearance of the LC (see, e.g., E. J. Poziomek, T. J. Novak, and R. A.MacKay (1973), “Use of liquid crystals as vapor detectors”, Mol. Cryst.Liq. Cryst. 27: 175-185).

FIG. 33 shows the basic principle of toluene detection using LC phasetransition. A sensor comprising LC supported on a substrate withpolymeric micropillars on a glass substrate initially appears bright(FIG. 33B, left). Upon exposure to toluene vapor, the LC materialundergoes a phase transition and the sensor appears dark (FIG. 33B,right). The pre- and post-exposure appearance of the LC sensor dependson the surface upon which the LC is deposited. For example, a LCsupported on glass substrate with polymeric micropillars (e.g., FIG. 33)initially appears bright and turns darker upon exposure to toluene whileLC supported on a polymer or other materials spin coated on a glasssubstrate (e.g., as disclosed below) initially appears dark and becomesbrighter as toluene causes a change in LC orientation due to polymerswelling or due to a change in the original surface.

LC Phase Transition

LC phases form as a consequence of intermolecular interactions thatstabilize the long range orientational ordering of molecules within theLC phases. These interactions can vary substantially (e.g., arising fromdipolar or steric interactions, dispersion forces, or hydrogen bonding)and depend on the structure of the molecules comprising the LC. When theLC material is exposed to an organic vapor analyte such as toluene, theanalyte partitions from the vapor into the LC and thereby perturbs theLC ordering, thus providing a measureable property of the sensor'sresponse. As such, the extent of the perturbation induced by the analytein the LC depends on the analyte-LC interactions, which will beinfluenced by factors such as relative polarities, polarizability, andhydrogen bond-donor/acceptor properties of the LC and analyte (see,e.g., S. J. Patrash and E. T. Zellers (1994), “Investigations of nematicliquid crystals as surface acoustic wave sensor coatings fordiscrimination between isomeric aromatic organic vapors”, AnalyticaChimica Acta 288: 167-177). Perturbation also depends on the shape(rod-like or planar) of the analyte. For example, arene-areneinteractions between substituted aromatic solutes (dopants) and anaromatic liquid crystal such as 4′-pentyloxy-4-cyanobiphenyl (SOCB)induce perturbations in the bulk properties of the liquid crystal phase(V. E. Williams and R. P. Lemieux (1998), “Role of dispersion andelectrostatic forces on solute-solvent interactions in a nematic liquidcrystal phase”, J. Am. Chem. Soc. 120: 11311-11315). As such,experiments were conducted to confirm that introducing an aromaticdopant (e.g., a VOC such as toluene) into the aromatic LC host (e.g.,5CB) causes a shift in the nematic-isotropic transition temperature thatis a function of dopant-host interactions.

During the development of embodiments of the technology provided,experiments were performed to evaluate two different LCs havingdifferent physical and chemical properties and to determine theireffectiveness for detecting toluene and other organic vapors. Thesensors fabricated for these experiments comprised a micrometer-thickfilm of LC (5CB or E7) supported on a glass substrate decorated withpolymeric micro-pillars (5 μm tall, 10 μm diameter, 20 μmcenter-to-center spacing). The micropillars are used to formmechanically robust thin films of the LC. The sensors were exposed totoluene or other organic vapor using an in-house gas exposure systemschematically shown in FIG. 40. The exposure system consists of a gasdelivery system (mass flow controllers, gas dilution system, etc.) andan optical image capture system (diffuse light source, CCD camera,polarizers, etc.). The saturated vapor of the target analyte wasgenerated by bubbling N₂ gas through the liquid analyte. The saturatedvapor analyte is then diluted at an appropriate ratio to generate thedesired concentration before delivering it to the exposure chamber thathouses the optical cells (sensor) prepared with the combination ofsubstrate and LC. This gas delivery system was used to deliver a rangeof gases at various concentrations from the ppb to the ppm range. Thesensors are placed inside an exposure chamber that is connected to thegas delivery system and flanked by two crossed polarizers. The chamberis placed between a CCD camera and a diffuse light source for real-timequantitative measurement of the optical change in the sensor.

Prior to exposure to toluene or other vapors, the LC possesses a brightappearance when viewed through crossed polarizers with a backlightsource. When the sensor was exposed to a known concentration of thetargeted analyte, the analyte diffuses into the LC film and lowers theisotropic transition to approximately room temperature. This exposureinduces a nematic-to-isotropic phase transition when a thresholdconcentration of analyte is reached (e.g., see FIG. 33). The phasetransition in the LC causes a striking change in the optical appearanceof the sensor. FIG. 41 shows a LC sensor (5CB) that was exposed to 5000ppm toluene vapor. Similar LC sensors exposed to either dry (RH 0%) orhumid (RH 95%) nitrogen alone didn't show any detectable changes. Thetoluene-induced changes of the LC are reversible—removing the vaporsupply restores the original bright appearance of the LC film The sensorresponses to toluene and other organic solvents are summarized inTable3.

TABLE 3 Organic solvent concentrations required to induce a phasetransition in two different LCs (5CB and E7) analyte 5CB concentrationE7 concentration benzene no change up to 20,000 ppm NA toluene 5000 ppm11,000 ppm m-xylene 1500 ppm NA nitrobenzene NA   200 ppm hexanes nochange up to 10,000 ppm NA isopropyl alcohol 7500 ppm NA formic acid20,000 ppm NA methanol 80,000 ppm NA

The data in Table 3 shows the relative sensitivity of LC sensors towardsdifferent organic solvents. These data suggest that the LC sensors madewith 5CB and/or E7 are relatively more sensitive to aromatic solventsthan non-aromatics (except for benzene). This is perhaps due tofavorable n-n interactions between aromatic solvents and cyanobiphenylLCs. The data in Table 3 also indicated that 5CB was more sensitive thanE7 due to its lower nematic-isotropic transition temperature.

The sensor response discussed above requires a very high concentrationof toluene to yield a visual change. A higher sensitivity can beobtained by: i) measuring the vapor response closer to the LC isotropictransition temperature, choosing LCs with a lower isotropic transitiontemperature, and using a sensitive instrument that will detect thesignal (e.g., the optical properties) prior to the formation ofisotropic phase.

Polymer Based Detection

Aromatic hydrocarbons (e.g., toluene) are known to swell variouspolymers. This effect is particularly pronounced for polymers of vinyland styrene moieties (see, e.g., P. Mullerbuschbaum, et al. (2006),“Fast swelling kinetics of thin polystyrene films”, Physica B, 385-386,703-705; B. Pejcic, et al (2007), “Environmental monitoring ofhydrocarbons: A chemical sensor perspective”, Env. Sci. & Technol.4(18): 6333-6342). LC orientation on a surface is extremely sensitive tothe physical and chemical changes that occur at the LC-surfaceinterface, thus the swelling property of polymers surface provides abasis for developing LC-based toluene sensors. Several polymers wereidentified (Table 4) based on the knowledge that these polymers swellupon toluene exposure.

TABLE 4 Exemplary polymers Structure Polymer Properties

Poly(vinyl acetate) [PVAc]: Average Mw ^(~)100,000; transition temp: Tg:30° C.

Polystyrene[PS] Average Mw ^(~)280,000; transition temp: Tg: 100° C.

Polystyrene[Low- MW PS] Average Mw ^(~)9500; transition temp: NA

Polyisobutylene[PiB] Average Mw ^(~)500,000; transition temp: Tg: ~64°C.

SC-F103 (Seacoast Science Inc.) NA

The selected polymers (Table 4) were screened for their activity withtoluene (e.g., at concentrations in the range of 1000 s of ppm) usingliquid crystal optical cell by pairing two glass surfaces to support athin film of LC. The polymers were deposited onto the clean glasssurface by spin coating. The detection of the polymer surface responseto the toluene vapor was conducted in two different ways. In the firstone, a polymer coated surface was first exposed to a known concentrationof toluene vapor followed by measuring the intensity of light passingthrough an optical cell made by combining the exposed polymer-coatedsurface, LC (E7), and an OTS-coated glass slide. The intensity of lightpassing through these optical cells was measured by using a polarizedmicroscope equipped with crossed polarizers and a backlight source. Achange of light intensity transmitted through the cell prepared with theexposed surface due a LC orientational change was compared with a cellmade with an unexposed surface. In the second method, an optical cellthat was made by a combination of a polymer-coated surface, LC (E7), andan OTS-coated glass slide was monitored for a change in the lightintensity transmitted through the cell placed between crossedpolarizers. Mean gray-scale intensity (MGSI) was measured as a functionof exposure time using a CCD camera and backlight source.

Polystyrene

The liquid crystal alignment properties of the polystyrene coated filmswere tested with glass-OTS sandwich optical cells. Glass slides (Fisherfinest: plain premium microscope slides) were thoroughly flushed withnitrogen (N₂) and then rinsed thoroughly with ethanol and acetone.Slides were initially dried with N₂ followed by heating at 100° C. for30 minutes. Slides were further UV-ozone cleaned for 5 minutes. Allglass slides used in this study were cleaned with this same procedure.The glass slides (1″×1″) were coated with 100 μl of 15 mg/ml stocksolution of the polymer polystyrene dissolved in toluene. In thissandwich optical cell, an OTS-coated glass slide and apolystyrene-coated glass slide were aligned facing each other. The twosurfaces were kept apart by a spacer having a thickness of ˜30 μm. A10-μl drop of LC (E7) in nematic phase was placed at the center of thepolymer-coated glass piece, then the OTS-coated glass piece was put ontop of it. The two surfaces, having a spacer and LC in between them,were held together using binder clips. The LC optical cells were imagedusing a polarizing optical microscope. A homogeneous alignment of E7 onthe polystyrene was induced by rubbing the polymer coated surface with avelvet cloth in one direction for five times before LC addition.Although the exact reason for this rubbing-induced LC alignment is notyet understood clearly, the technique is used very often to align LCs ondifferent polymer surfaces (Wu, et al. (1996) “Liquid-crystal alignmentof rubbed polyimide films: A microscopic investigation”, Applied PhysicsB: Lasers and Optic, 62(6), 613-618).

FIG. 42 shows microscope images of cells prepared with the rubbed orunrubbed polystyrene films and E7 viewed at different orientations ofthe cell with respect to the cross polarizers. FIG. 42b shows a changein light intensity (e.g., from dark to bright) through the optical cellmade with rubbed polystyrene film when the cell was positioned at 0 and45 degrees with respect to the crossed polarizers. This change indicatesa homogeneous planar alignment of the LC on the rubbed polystyrenesurface. Similar rubbing-induced homogeneous alignment was observed withPVAc, PiB, and SC-F103. Quite unexpectedly, the surface prepared withlow MW PS from a 15 mg/ml toluene stock solution showed homeotropicalignment without any rubbing (FIG. 42c ). As this surface aligned LC(E7) homeotropically with or without rubbing they were not furthertested with toluene. However, the same low MW PS was used to test thedewetting induced orientation of LC (see below).

For toluene exposure studies, a series of optical cells were preparedusing mechanically rubbed PS substrates coated on glass, an OTS-coatedglass slide, and E7. The cells were exposed to nitrogen or toluene andimages were collected at a regular time interval using the experimentalset up shown in FIG. 40. The results of exposure experiments aresummarized in FIG. 43. The data in FIG. 43a show that there was nochange in the overall planar alignment in the LC cell due to tolueneexposure when viewed under the microscope. However, the data in FIGS.43b and 43c show a definitive change in the LC cells due to tolueneexposure when compared to the cell exposed to nitrogen alone. A similarchange of a smaller magnitude was observed when the LC cell was exposedto 2900 ppm toluene vapor.

In another test, a PS coated surface was used where the polymer surfacewas first exposed to a known concentration of toluene vapor followed bymeasuring the intensity of light passing through an optical cell made bycombining the exposed polymer coated surface, LC (E7), and an OTS coatedglass slide. A series of glass substrates were coated with PS films, thesurfaces were mechanically rubbed, and then exposed to toluene. Aftertoluene exposure, the surfaces were overlaid with LC (E7) to prepare anoptical cell for measurement. A sandwich cell prepared in this mannerwas exposed to 5020 ppm toluene for 1 hour and was observed to have thesame planar alignment as the cell made with unexposed surfaces. Asimilar cell made with a surface exposed to saturated toluene vaporinitially showed a random alignment but slowly changed to a planaralignment. These observations suggest perhaps the change due to tolueneexposure on rubbed PS surface is transient and quickly disappears astoluene supply was ceased.

Poly(Vinyl Acetate)

A series of glass substrates were coated with poly(vinyl acetate) [PVAc]films; then, the surfaces were mechanically rubbed, exposed to nitrogenor toluene, and then overlaid with LC (E7) to prepare an optical cellfor measurement. Clean glass slides (1″×1″) were coated with 100 μl of16 mg/ml stock solution of the PVAc polymer in a toluene solution. Itwas observed that a surface coated with PVAc promoted a random alignmentof LC. When this surface was exposed to a saturated vapor of toluene(e.g., at 28,680 ppm) followed by LC overlay no change was observed inthe LC alignment. A homogeneous alignment of E7 on the PVAc-coated filmswas induced by rubbing the film surface with a piece of velvet cloth inone direction five times (e.g., as used for PS in the experimentsdescribed above). FIG. 44 shows images of LC cells prepared withPVAc-coated surfaces viewed at different azimuthal orientations of therubbing directions with respect to the cross polarizers.

The data in FIG. 44 showed that a rubbed PVAc surface yielded a changein the LC alignment upon exposure to saturated toluene vapor (28,680ppm). Unlike rubbed PS-coated surfaces, the random LC alignment did notrevert to a planar alignment quickly. A reduction in the tolueneconcentration to 8605 ppm did not result in any difference in the LCalignment when compared to an unexposed surface. To avoid LC additionafter toluene exposure an optical cell was prepared with a rubbed PVAccoated surface, LC (E7), and an OTS coated glass slide. The cell wasexposed to 8605 ppm toluene for 20 hours but no detectable differencewas observed due to toluene exposure. To examine the effect of rubbingon the toluene response at sub-saturation concentrations, a series ofPVAc-coated surfaces were prepared using less rubbing and exposureexperiments were conducted with sub-saturated concentrations of toluenevapor. FIG. 45 shows images of LC cells viewed at different azimuthalorientations of the rubbing directions with respect to the crosspolarizers. The data in FIG. 45 showed that by reducing the rubbing thesensitivity towards toluene was significantly enhanced. Initialexperiments suggested that a thinner polymer coating (e.g., coating froma 5 mg/ml polymer stock solution) and minimum rubbing improve thesensitivity of toluene detection using rubbed polymer surfaces. Theimages in FIG. 45 showed a very large change in the transmitted lightintensity through the optical cells upon exposure to 8600-2900 ppmtoluene vapor.

SC-F103

SC-F103 is a hyperbranched fluoroalcohol polycarbosilane. Experimentswere conducted to test SC-F103 in LC-based toluene detection. Thepolymer was spin coated on clean glass surfaces from a 16 mg/ml stocksolution of 1:1 toluene and CHCl₃. Optical cells were then prepared withthe polymer-coated surface and OTS as described before. Images of cellsare shown in FIG. 46. As the glass surface was coated with the polymer,there was no visible indication that the polymer coating was formed onthe glass. However, cells prepared with uncoated glass showed adifferent alignment pattern than the SC-F103 coated cells (FIG. 46A, aand b). This suggests the presence of a film of polymer after spincoating. Interestingly, the surface prepared with SC-F103 showedhomeotropic alignment without any rubbing. This is similar to theobservation made earlier with the cells prepared with low MW PS.

FIGS. 46B and 46C show images of SC-F103 cells before and after exposureto toluene. Only a small change was observed before and after exposure.However, a similar change (e.g., bright patches that turned darker uponexposure) was also observed when a similar SC-F103 cell was exposed todry nitrogen alone. This change in part of the bright patch may arise asa result of the formation of LC-polymer homogeneous mixture over time tolower the phase transition temperature in LC.

Hydrocarbon Based Detection

As the LC orientation is very sensitive to the surface composition it islikely that the dissolution of toluene-soluble materials coated onto thesurface will initiate a change in the LC orientation. It was anticipatedthat the toluene solubility of materials coated as a thin film on asurface that is known to align LC in a pre-defined direction (e.g.,rubbed polymer, OTS treated surface, etc.) would dewet the surface, thuslocally exposing the underlying surface to the LC and leading toreorientation of the LC. Long chain aliphatic hydrocarbons (e.g., C₁₈and higher) are solid at ambient temperature and possess a very highsolubility (>100 mM) in toluene. Initially, four hydrocarbons as shownin FIG. 47 were chosen for these studies.

A glass piece was coated with the neat hydrocarbon for testing. Solidoctadecane was first spread onto the glass piece (1″×3″) with a spatulaand then another glass piece was placed on it. The hydrocarbonsandwiched between the two glass pieces was heated to ˜50° C. The meltedoctadecane spread into the cavity. The top glass slide was then draggedfrom one end to the other to spread the liquid on the base glass pieceto form a thin film Upon cooling, a white film was observed on theplate. The octadecane-coated glass surface was then cut into smaller1″×1″ pieces and E7 was spin coated on the hydrocarbon layer. Images areshown in FIG. 48.

One of the 1″×1″ pieces with LC coated on a hydrocarbon layer wasexposed to 5020 ppm toluene using the exposure system shown in FIG. 40.The result of this exposure experiment is shown in FIG. 49. The datashown in FIG. 49A indicated a change from bright to dark as exposuretime increased. However, this change was transient, e.g., the exposedchip was gradually turned towards a brighter appearance as the toluenesupply was ceased after 19 hours. Though the images acquired beforeexposure and after toluene exposure had been ceased for 19 hours (FIG.49B) were not identical, the nature of the changes observed indicate anLC phase transition due to toluene exposure.

To investigate the origin of this change, solid octadecane was depositedfrom its chloroform solution onto two OTS-coated glass slides and thenpaired with another OTS-coated piece to form optical cells. In one ofthe cells, the cavity between the OTS-coated pieces (2×35 μm=70 μm) wasfilled with E7 and the other cell was left untreated. Both of the cellswere individually exposed to 5020 ppm toluene vapor and images werecollected at a regular interval during a total period of 5 hours ofexposure. The data from this exposure experiment are shown in FIG. 50A.The data in FIG. 50A(i) show that the areas where both LC and octadecanewere present (the bright spots in the image) change to a darkerappearance over time. However, the optical cell containing onlyoctadecane spots (FIG. 50A(ii)) appear relatively less bright undersimilar conditions (FIG. 50A(i)) but undergoes no detectable changesupon 5020 ppm toluene exposure for 5 hours. This observation suggeststhe changes are associated where LC is in contact with the octadecane.

In another experiment, 10 μl of E7 and E7 mixed with octadecane wereplaced on a piece of glass piece to test the bulk properties of theoctadecane-E7 mixture. The spotted glass piece was then placed insidethe experimental chamber and exposed to 5020 ppm toluene for 2.5 hoursand images were collected. At the end of toluene exposure the gas flowwas turned off but the image collection was continued for another 0.5hour. The data are shown in FIG. 50B. The data in FIG. 50B show that thedark appearance of the LC-octadecane mixture that resulted from 2.5hours of 5020 ppm exposure begins to turn brighter quickly after the gaswas turned off. Under the same conditions, the E7 spot remainedunchanged throughout this 3-hour period indicating that the bright todark transition associated with the E7-octadecane mixture is due to thelowering of the nematic to isotropic (N-I) phase transition temperatureof the mixture. This lowering of the N-I transition temperature in E7was the result of soluble octadencane in E7.

Another long chain hydrocarbon (tetracosane, C₂₄) was also tested. AnOTS-coated glass piece was first coated with the neat hydrocarbonfollowing the similar procedure used in coating octadecane on glass(FIG. 48A). The tetracosane-coated surface was then cut into smallerpieces to make optical cells, paired with another uncoated OTS piece,and the cavity filled with LC (E7). The LC (E7) laid onto thehydrocarbon film sandwiched between two OTS pieces aligned LChomeotropic. However, the thickness of the hydrocarbon may not have beenthe same throughout the plate as suggested by the appearance of somebright patches that were observed in the cells. One such cell wasexposed to 5020 ppm toluene for 43 hours (FIG. 51). The white patchunder the yellow circle disappeared with long exposure to toluene.However, it started to reappear slowly as the cell was taken out of theexposure chamber indicating a phase transfer due to toluene exposure.

Summary

Experiments were conducted to test various LC films and LC alignments onpolymer and other surfaces. The data provide an understanding of sensorsubstrate features and how they can be engineered to affect sensorresponse time for the detection of toluene. The LC phase transitionoffers a simple technique for the detection of toluene and other organicsolvents. The detection threshold varies over a wide range ofconcentrations based on analyte characteristics and the LC used.

Polystyrene (PS) was identified as a suitable polymer for the detectionof toluene using a change in LC orientation. It is contemplated that thetechnology will detect toluene in the range of from 50 ppm to 3000 ppm.The speed at which the sensors respond to toluene vapor can becontrolled by fabricating sensors on polymer coated substrates with adefined thickness, controlled rubbing, and a cell configuration thatallows unrestricted toluene flow. C₁₈ and C₂₄ hydrocarbons were testedto improve the sensitivity of toluene detection. These highlytoluene-soluble hydrocarbons facilitate toluene detection by loweringthe nematic-isotropic phase transition temperature of the LC used.

Example 9 Detection of Volatile Organic Compounds Using PolymerDispersed Liquid Crystals

During the development of embodiment of the technology provided herein,experiments were performed to test feasibility of using a polymerdispersed liquid crystal (PDLC) system for detection of volatile organiccompounds (e.g., toluene). The results from these preliminaryexperiments suggest that PDLCs detect VOCs such as toluene if the PDLCsare formed with a polymer that is known to adsorb VOCs. The sensitivityof detection depends on a number of parameters that influence themorphology of the PDLCs. Data collected show that toluene at 2000 ppm isdetected using PDLCs prepared from LC E7 and polystyerene (PS) usingsolvent-induced phase separation. It is contemplated that detection willbe more sensitive (e.g., detect lower amounts of VOC) with furtheroptimization, e.g., optimizing parameters to improve the sensitivity ofdetection, and providing more precise PDLC morphology by better controlof the fabrication process. To increase the dynamic range of detection,PDLCs were formed on rubbed polymer surfaces.

Background

Initial experiments with pure liquid crystals 5CB and E7 demonstrated adetection of ˜5000 ppm and ˜12,000 ppm, respectively (see Table 3). Toimprove the sensitivity of detection, a number of approaches have beendiscussed including confinement of LC in small cavities formed frompolymeric materials that are known to adsorb toluene. The basic ideabehind this approach relies on the fact that the LC confined inside asmall cavity assumes some initial director configuration. Once thisconfined LC is exposed to VOC such as toluene, the polymer undergoesstructural changes that will induce a change in the ordering of LCinside this cavity.

FIG. 34 shows a basic principle behind this approach of detection wherea LC droplet is confined inside a polymer matrix that is known to adsorbthe target VOC. The boundary condition is such that the orientation atthe LC-polymer interface provides alignment of the LC in an orientationthat is perpendicular to the interface. The polymer matrix is confinedso that it can deform only along one direction. Upon exposure to theVOC, the polymer undergoes deformation that subsequently changes theshape of the LC droplet and the orientation of LC components. Differentembodiments of the same basic principle are envisioned. One simpleformat that was tested made use of well-defined PDLC structures thatdeformed and/or changed upon exposure to toluene. Initial experimentswith the PDLCs formed by mixing a UV-curable polymer (NOA-21) and E7showed that the sensors respond to toluene but the response was not dueto deformation of the polymer structure but was more related to thedecrease in the nematic-isotropic transition temperature of the E7 as aresult of dissolution of uncured monomer into the LC E7. Since NOA-21forms a strong polymer upon exposure to UV light the polymer matrix didnot deform upon exposure to toluene. However, the uncured monomersdissolved in the LC were sufficient to lower the phase transitiontemperature of E7 from ˜65° C. to a temperature close to the phasetransition temperature of 5CB (˜35° C.). As such, the sensorsdemonstrated a response to ˜5000 ppm toluene that was similar to theresponse of a pure 5CB sensor. In the experiments described below, PDLCswere formed using polymers that were known to dissolve in toluene (e.g.,polymers such as polystyrene and polyvinylacetate that are known toadsorb toluene) to prepare PDLCs with the anticipation that they woulddeform upon exposure to toluene. Embodiments are envisioned in whichPDLC is prepared in a strained configuration so that exposure to toluenewill release the strained energy for sensitive detection of toluene.

High Molecular Weight Polystyrene

A polystyrene (PS) solution of approximately 2% by weight in toluene wasprepared by mixing PS (average MW 280,000, Sigma Aldrich #182427) intoluene, then vortexing and heating the solution to 70° C. in an oven.Similarly, a ˜2% E7 solution was prepared by dissolving LC E7 in tolueneand vortexing the solution. These two stock solutions were used toprepare PDLCs with different E7:PS ratios. Using the stock solutions,the

PS:E7 solutions were mixed at a 1:2 ratio. Glass slides were coated withOTS with masked 7-mm diameter areas. These areas were cleaned and then50 μl of the 1:2 PS:E7 mixture was deposited and the solvent was allowedto evaporate slowly by covering with a petri dish. After completesolvent evaporation, the LC separates from the polymer matrix formingwell-defined droplets (FIG. 52a ). These droplets scatter light andappear bright when viewed between crossed polarizers. When thesedroplets were exposed to toluene vapor generated by bubbling dry N₂ gasthrough liquid toluene (inside an exposure chamber at a flow rate of 200ml/min using set up similar to FIG. 40) the toluene vapor thus producedinduced phase transition in E7 and the PDLC film appeared dark (FIG. 52c). After establishing that the PDLC film gives a response to toluenevapor at a high concentration using an optical microscope, these sensorswere then exposed to ˜8600 ppm toluene vapor using the exposure systemsimilar to FIG. 40. The optical response behavior of these sensors totoluene vapor is shown in FIG. 52d . The results indicate that the PDLCsensors are more sensitive to toluene vapor than pure E7 sensors. Thesefindings suggest that by incorporating the polymer into the

LC materials the partition of toluene vapor into the LC film is enhancedand therefore improves the sensitivity. However, the sensitivity ofdetection was sub-optimal. Additionally, the PDLCs formed using thisapproach yielded spherical droplets without any stored elastic strain.

Low Molecular Weight Polystyrene

Lower molecular weight PS rapidly phase separate forming well-definedPDLCs. Polystyrene with a lower molecular weight (e.g., MW 20,000, SigmaAldrich #327743) was tested to demonstrate the feasibility of detectionof toluene using PDLCs. PDLC droplet were formed on 0.8 cm×0.8 cm glasssubstrates. The glass substrates were thoroughly rinsed with acetone andethanol then dried in a stream of nitrogen. These substrates weresubjected to a 2 minutes UV ozone treatment. PDLC droplets were formedon these substrates by depositing 15 microliter of 1:2 (PS: E7) mixtureprepared by dissolving both PS and E7 in toluene forming 2% mixtures. Togenerate the PDLCs with ˜2 micrometer diameter, the solvent was allowedto evaporate slowly. The size of the PDLC droplets was confirmed byusing polarizing optical microscope.

Results Effect of Thermal Annealing on the Morphology

As discussed above, the initial morphology of the PDLCs formed by SIPdepends on a number of parameters. Among these, thermal annealing afterformation of the PDLCs has an effect on the size of the droplets. Forthe LC-PS composition examined during these studies, the PDLC dropletsize appears to increase after incubation. In these experiments, PDLCswere formed using the above protocol, and polarizing optical microscopicimages were taken before incubation. After incubation at 60° C. for 4minutes and equilibration at room temperature, the morphology of thePDLC was imaged. These sensors were then exposed to 8600 ppm toluene for10 minutes and the morphology was imaged again. The results are shown inFIG. 53. The results show that the morphology of PDLCs can be controlledafter formation. Additionally, the results also show that the dropletscoalesce after exposure to toluene.

Effect of Toluene Concentration on Response

Identical sensors with 15-μl volume and 4-minute incubation at 65° C.were prepared following the protocol described above. These sensors werethen exposed to toluene at different concentrations. Differentconcentrations of toluene were generated by mixing a saturated vapor oftoluene (generated by bubbling dry N₂ through liquid toluene) at anappropriate ratio with dry N₂. Six sensors were first exposed to 4000ppm toluene for 5 minutes. Before the 5-minute exposure to toluene, eachsensor was placed inside the exposure chamber and was allowed toequilibrate for 30 seconds. After the sixth sensor was exposed for 5.5minutes, the concentration of toluene was increased to 8000 ppm whilethe response was still measured. After a 1-hour equilibration, six moresensors were exposed to 8000 ppm in a similar fashion. After the 12thsensor (the sixth sensor exposed to 8000 ppm) was exposed to 8000 ppmfor 5.5 minutes, the toluene vapor was turned off and the N₂ flow ratewas increased to 200 sccm. The sensor was exposed to dry N₂ during thisequilibration time. After a 45-minute equilibration, three sensors wereexposed to dry N₂. FIG. 54 shows a normalized response as a function ofexposure time. The results indicate that the sensors respond to 8000 ppmtoluene. Although the sensors exposed to high concentration of toluenereverse back to some extent, there is some permanent change in thesensor. The response from 4000 ppm is very similar to the response fromdry N₂. The small response to dry N₂ suggests that the PDLC dropletsrespond to air flow.

To determine whether the small response of the sensor to dry N₂ was aresult of flow, multiple experiments that included sequential exposureto dry N₂ and 4000 ppm toluene were performed under differentconditions. One representative exposure result is shown in FIG. 55. Inthis experiment, the sensor was first equilibrated inside the exposurechamber for 30 seconds and then exposed to dry N₂ for ˜20 minutesfollowed by a ˜20 minute exposure to 4000 ppm toluene. Finally, thesensor was exposed again to dry N_(2.) The results suggest that thesensor responds to dry N₂ alone. However, the response induced by dry N₂is relatively smaller than that from 4000 ppm toluene. This resultsuggests that this particular embodiment of the sensor has a limit ofdetection for toluene that is around 4000 ppm.

PDLCs on Rubbed Polymer Films:

The LC directors inside the PDLCs formed on an untreated surface arerandomly distributed before exposure to toluene and they remain randomlydistributed after they coalesce to form larger droplets. This indicatesthat the PDLCs undergo a transformation from a higher scattering stateto a lower scattering state. If small PDLC droplets are formed on arubbed surface they scatter light (e.g., appearing bright).Additionally, these droplets coalesce and form larger domains that arealigned by the underlying rubbed surface. To test this hypothesissurfaces were coated with nylon (Evalmide 8061, Dupont Chemicals) andmechanically rubbed. PDLC droplets were formed as described below.

A solution of 0.2% Evalmide 8061 (Dupont Chemicals) was prepared bydissolving an appropriate amount in methanol. The 1″×1″ aluminosilicateglass substrate was pre-scribed to 0.8 cm×0.8 cm and was thoroughlyrinsed with acetone and ethanol then dried in a stream of nitrogen. Thesubstrate was subjected to a 2-minute UV ozone treatment. A film ofEvalmide was formed by spin coating the Evalmide solution on to thissubstrate. After evaporation of the solvent, the film was mechanicallyrubbed 10 times using the weight of the rubbing cloth by placing thesubstrate between two AlSi glass slides. A mixture of 2:1 LC-polystyrenewas prepared by mixing 2% solution of E7 and polymer at an appropriateratio. 180 microliters of LC-polymer mixture was dispensed on the rubbedsubstrate. The evaporation rate of the tolune was controlled by coveringthe substrate with a petri dish with a small (˜5 mm) opening. Aftercomplete solvent evaporation the sensors were broken into individualpieces and imaged using polarizing optical microscope to determine thesize of droplet. In some embodiments, the sizes of the droplets werecontrolled by incubating these sensor chips at different temperatures asdescribed above.

Results Morphological Changes Upon Exposure to Toluene

Sensor chips were prepared following the above protocol. The sensorswere then exposed to different concentrations of toluene (2000, 5000,and 8000 ppm at 45% RH) for ˜10 minutes while the dynamic response wasrecorded. The morphological appearance of PDLC droplets were imagedbefore and after exposure at both 0° and 45° orientations using apolarizing optical microscope. FIG. 56 shows the optical images of PDLCdroplets before and after exposure. The images show that before exposureto toluene vapor small “domains” of LC are aligned along the rubbingdirection. In addition, most droplets are separated and scatter light.As a result, the brightness does not significantly change upon rotationof the sensor chip between crossed polarizers. When the sensors areexposed to toluene vapor, the PDLC microdroplets coalesce to form larger“domains” that are aligned along the rubbing direction of the underlyingsurface. As a consequence, there is less scattering and the sensorappears much darker if the rubbing direction is parallel to thetransmission axis of the polarizers. The images also show that thedroplets coalesce more efficiently at a higher concentration than at alower concentration. As a consequence, the domains are larger at ahigher concentration than at a lower concentration. A prolonged exposureto a high concentration leads to formation of isolated droplets on thesurface or dewetting of the LC film on the surface (see below).

To establish that the sensor response did not result from a lowering ofthe phase transition temperature due to dissolved PS molecules in the LChost, a sensor chip was first exposed (with the rubbing directionparallel to the transmission axis of the polarizer) to 8000 ppm toluenefor 8 minutes until the sensor appeared dark between crossed polarizers.The exposure chamber was then opened and the sensor was rotated by 45°so that the rubbing direction was at a 45° orientation with respect tothe polarizers. Since the LC domains were aligned along the rubbingdirection the sensor appears bright. In this orientation, the sensor wasexposed to 8000 ppm toluene for 8 minutes. Upon exposure, the sensorbecame darker and more dark spots reappeared. However, the majority ofthe sensor still remains bright indicating that the LC within thesebright domains is still in a nematic phase and the response seen in thefirst exposure is not due to lowering of the phase transition. Theresponse is indeed due to merging of the microdroplets that subsequentlywere aligned along the rubbing direction that produced the darkappearance (FIG. 57). In fact, one of the sensors was heated on a hotplate and the morphological changes were observed in real time as thetemperature was raised. Although the PDLCs seem to disappear at 54° C.,no phase transition was observed until the sensor chip was heated to 65°C. This observation indicated that the change in the optical appearanceof the PDLC droplets upon exposure to toluene is not a result oflowering the phase transition temperature.

Optical Response

Multiple sensors were exposed to different concentrations of toluene andnitrogen both at 45% RH. The optical responses for some representativesensors are shown in FIG. 58. The data show that these sensors detecttoluene at a concentration as low as 2000 ppm. The response time (thetime it takes for the sensor response to “equilibrate”) depends on theconcentration of toluene. It should be noted that the sensors show ahigher contrast ratio (e.g., a large change in the optical response)compared to those prepared without rubbed surface (see FIG. 58). Someinconsistency of the sensor response may have resulted from differencesamongst the 9 different sensor ships produced from the same 1″×1″substrate. For example, the evaporation pattern of toluene may have beenslightly different among different sensor chips produced from the samesubstrate.

These results indicate that the E7 PDLCs supported on a rubbed surfaceare more sensitive than pure E7. The presence of the underlying rubbedsurface aligns the LC domains formed after exposure to toluene. Thisapproach of forming PDLCs on a rubbed surface provides for the detectionof toluene at concentrations as low as 2000 ppm at 50% RH.

As these initial experiments suggest, humidity has some effect on thesensor response. Finally, it is contemplated that the sensitivity may beimproved by using low molecular weight polymers that facilitatemorphological changes of the PDLC droplets at lower concentration.

Example 10 Detection of Volatile Organic Compounds Using De-WettingInduced Orientational Transition of LCs.

Various approaches have been contemplated to detect volatile organiccompounds (VOCs) such as toluene. Some embodiments comprise detectingthe dewetting of a thin polymer film upon exposure to an analyte byusing liquid crystals (LCs). To test this approach, experiments wereperformed to demonstrate that a polymer film such as a polystyrene (PS)formed on a glass substrate absorbs toluene and dewets the surface uponexposure to toluene. This dewetting of the PS film is reported in someembodiments using an LC film supported on the PS. Using this approach,detection of toluene at 2000 ppm was demonstrated using a micropillaredsubstrate cleaned with oxygen plasma. It is contemplated that additionalexperiments and optimization will result in establishing a lower limitof detection and further characterizing this method of detection ofVOCs.

Background

The stability of a polymer film on a substrate depends on a number ofparameters such as the surface energy of the substrate, the physical andchemical structure of polymer material, the thickness of the film, etc.Some polymer materials such as polystyrene form a stable film on solidsurfaces such as such as glass or silicon. PS films dewet if thethickness of the film is below a critical value or the temperature iselevated above a critical temperature. Exposure to VOCs (e.g., toluene)and heat affect a polymer film similarly—e.g., exposure to toluenelowers the glass transition temperature of the polymer or it can inducean orientational change of LC supported on rubbed polymer films. Assuch, it was anticipated that exposure to toluene would induce thedetwetting of the film Then, if the orientation of LC on the polymerfilm and on the substrate is different, then exposure to VOCs would leadto a change in the orientation of the LC and therefore provide a meansto detect the presence of VOCs. This approach is particularly usefulwhen the polymer material to be used not only provides a LC orientationdifferent than the underlying substrate, but also absorbs VOCs. Theseexperiments are based on the earlier findings that showed low molecularweight PS (MW ˜10000) films coated on glass substrates align LC such asE7 perpendicularly to the surface and that the polymer dispersed liquidcrystal droplets formed with PS start dewetting the surface when exposedto a high concentration (˜8000 ppm) of toluene for prolonged time. Thebasic principle behind the dewetting-induced orientational transition isshown in FIG. 35. The main difference between an analyte-inducedorientational transition and this approach is that in theanalyte-induced orientational transition approach the LC remains incontact with the chemically functionalized surface that gets modifiedupon exposure to target analytes. In this approach, the chemicallyfunctionalized surface (in this case polymer) is modified and physicallydewets the substrate and, as a result, the LC comes in contact with theunderlying surface.

Alignment of LC on PS Coated Surfaces and Effect of Thermal Annealing

A PS solution (˜2% by weight) was prepared by dissolving PS (MW 10,000,Sigma 81406-1G) in of toluene at appropriate ratio. The solution wasthen repeatedly heated at 70° C. and vortexed until all the PS wascompletely dissolved to give a clear solution. This solution was used asa stock solution to coat different substrates forming PS films.

Surfaces coated with polymers such as polyimide (PI) and PS are widelystudied for LC alignment. While PI-coated surfaces are widely used indisplay industries to give planar and homeotropic alignment, PS coatedsurfaces are studied because of their uniqueness in LC alignmentproperties. Untreated surfaces coated with high molecular weight PS arewell known for their ability to promote random planar alignment.However, these surfaces, when mechanically rubbed, provide LC alignmentthat is perpendicular to the rubbing direction. PS coated surfaces,although not stable for display applications, are widely studied becauseof this unusual alignment behavior. Rubbed polyimide surfaces, incontrast to rubbed PS surfaces, provide a uniform alignment along therubbing direction. While most studies on rubbed PS surfaces have beenperformed with high molecular weight PS, systematic studies of LCalignment on surfaces treated with PS with different (e.g., lower)molecular weights have recently started to emerge (see, e.g., Seung WooLee, et al. (2003), “Effect of Molecular Weight on the SurfaceMorphology, Molecular Reorientation, and Liquid Crystal AlignmentProperties of Rubbed Polystyrene Films”, Macromolecules 36: 9905-9916).

Experiments performed during the development of embodiments of thepresent technology suggested that surfaces coated with low molecularweight (MW˜10,000) PS promotes alignment perpendicular to the surface,that is, homeotropic alignment. To confirm this observation, several 1″x 1″ glass substrates (Fisher Scientific) were cleaned by rinsing withacetone and ethanol. The substrates were then dried in a stream ofnitrogen. Dry surfaces were UV-ozone cleaned for 2 minutes. Somesubstrates were set aside to prepare LC cells with untreated glass,while other substrates were coated with a film of PS by spin coating thestock PS solution at 2000 rpm. After spin coating, the solvent wasallowed to evaporate and the substrates were broken into 0.5″×1″ pieces.These substrates were used to assemble LC cells with different substratecombinations:

-   -   1. PS-OTS: PS coated substrate paired with OTS treated        substrate; cell gap ˜30 micron    -   2. PS-PS: PS coated substrate paired with PS coated substrate;        cell gap ˜25 micron defined by mylar    -   3. Glass-OTS: Clean glass substrate paired with OTS treated        substrate; cell gap ˜30 micron    -   4. Glass-Glass: Clean glass substrate paired with clean glass        substrate; cell gap ˜25 micron defined by mylar

A 10-microliter droplet of E7 was placed at the center of one of thesubstrates and the top substrate was overlaid to form a LC cell byholding the two substrates together (e.g., with binder clips). Thesecells were then imaged between crossed polarizers using a digital cameraand then using the polarizing optical microscope (POM) to determine thealignment of the LC. Conoscopic images of the cells were also recordedto confirm the homeotropic alignment of the LC. After imaging, thesecells were incubated (e.g., inside an oven) at 60° C. for 10 minutes.The cells were them imaged for LC alignment. FIG. 60 shows the cameraand POM images of the LC cells fabricated with different surfacetreatment conditions.

The optical images show that the clean glass substrates, whether pairedwith another clean glass substrate or with an OTS-treated surface,provide a planar alignment (bright image) on the surface and thealignment remains unchanged upon thermal annealing. A surface treatedwith PS (MW˜10,000) provides homeotropic alignment (dark images)independent of whether it is paired with an OTS treated surface or a PScoated surface. The “Maltese Cross” shape observed in the conoscopicimage confirms homeotropic alignment. Unlike other high molecular weightPS films, this PS provides homeotropic alignment. When these LC cellswere incubated at 60° C. for 10 minutes, the cells prepared with PS filmappeared bright between crossed polarizers. However, the LC cellsprepared with the clean glass substrates remain unchanged (stayedbright). This indicates that the LC alignment behavior on the PS treatedsurface changes after thermal annealing.

A closer look at the microscopic images reveals that small islands of LCdomains form on the LC cell after thermal annealing (FIG. 59). The sizeand number of these domains depend on the extent of thermal annealing.The planarly aligned patterns randomly originate from nucleation sites,very similar to the patterns observed in typical dewetting of a polymerfilm on a solid surface. Formation of these domains during thermalannealing indicates that the LC reorientation observed in theseexperiments is a consequence of PS film dewetting on the surface and isnot from molecular reorganization within the film itself. When these PScoated films were mechanically rubbed, the cells appear bright. Thebrightness of the LC cell depends on the rubbing strength and relativeorientation of the cell with respect to transmission axes of the crossedpolarizers, indicating that the rubbed PS film can provide uniformplanar alignment if the surface is rubbed. However, after annealing, thecells remain bright and the microscopic images show random planaralignment (FIG. 60).

Exposure to Toluene

To test whether these observations of thermal annealing apply to sensingtoluene vapor, a 1″×1″ glass substrate was pre-scribed into 9 0.8 cm×0.8cm pieces. The scribed side of the substrate was then cleaned and coatedwith a 2% PS solution as described above. After solvent evaporation, thesubstrate was broken into 9 individual 0.8 cm×0.8 cm pieces. Each piecewas then coated with 20 microliters of LC E7 to fabricate a sensor chip.The sensor chip was then imaged using the POM. After imaging, the sensorwas exposed to saturated toluene vapor by holding it for ˜30 seconds inthe head space above a 1-ml volume of toluene at the bottom of an 8-mlvial. The sensor was imaged again using the POM. As shown in FIG. 61,the sensor appeared bright upon exposure to saturated vapor of toluene.This result shows that the PS coated surface can be used to detecttoluene based on dewetting-induced orientational transition of LC.

To test detection toluene concentrations lower than a saturatedconcentration of toluene, an identical chip was exposed to 5000 ppmtoluene at 50% RH using the exposure system. A toluene concentration of5000 ppm was generated by mixing a saturated concentration of toluenewith wet nitrogen to provide the appropriate ratio. The sensor wasexposed inside a small exposure chamber at a flow rate of 200 ml/minwhile the optical appearance of the sensor chip was recorded in realtime. FIG. 62 shows the macroscopic and POM images of the sensor chipafter exposure to 5000 ppm toluene. These results indicate that there isa noticeable change in the LC alignment upon exposure to 5000 ppmtoluene. The microscopic images also show that the texture of thedefects induced by exposure to toluene is very similar to that inducedby thermal annealing. This suggests that the mechanism responsible forthe change in the optical appearance due to thermal annealing is verysimilar to that due to exposure to toluene.

Sensors on Micropillared Substrates

Micro-pillared sensor substrates with 4 sensors (plasma cleaned using100 W energy for 30 seconds) were rinsed briefly with acetone andethanol. The chips were then dried in a stream of nitrogen. These chipswere cleaned using UV-ozone for 2 minutes. The chips were coated with 2%PS using the same protocol described above. After solvent evaporation,the sensors were filled with a small amount of LC E7 using a tip of apaper clip until the sensor was filled with LC.

The alignment of LC on these sensor chips was weakly homeotropic in thesense that there were few white lines with planar alignment in anotherwise homeotropic dark background (FIG. 63). After storing thesensors overnight, they were exposed to nitrogen (at 50% RH for 30minutes) and 2000 ppm toluene (at 50% RH for 4 hours) at 200 ml/mininside a small exposure chamber. During the 30 minute exposure, nosignificant change was observed in the appearance of the sensor exposedto nitrogen. However, the sensor exposed to 2000 ppm exhibited adetectable change in the appearance at 30 minutes. To confirm that thechange resulted from exposure to toluene at 2000 ppm, the sensors wereexposed for a total of 4 hours. At the end of 4-hour exposure, thesensor clearly appeared different. These images were also analyzed toquantitate the sensor response. The plot in FIG. 64(f) shows that thesensors respond to 2000 ppm toluene.

Summary

Experiments performed with a thin film of LC supported on surfacescoated with low molecular weight PS confirmed that the LC E7 adopts ahomeotropic alignment on these surfaces. This orientation of LC isindependent of the nature of the second interface (PS, OTS, or air)used. Results show that when a LC film supported on a PS coated surfaceis exposed to saturated toluene vapor, it undergoes an orientationaltransition and the film appears bright between crossed polarizers. Thisbehavior is similar to that observed upon a thermal annealing ofpolystyrene film At a lower concentration, the film shows a partialorientational transition that appears as a randomly aligned domainoriginating from nucleation sites in a homeotropic background. Theseresults suggest that the orientational transition is associated with thedewetting of the PS film on the glass substrate. Using plasma cleanedmicropillared substrates, 2000 ppm of toluene was detected. To exploitthis mechanism of detection of toluene and other VOCs, it iscontemplated that parameters such as the surface properties and thethickness of the PS film, etc. will have a role in improving the sensorsensitivity and response.

Example 11 Comparison of Microfluidic Cells Fabricated with DifferentMetal Salts

During the development of embodiments of the technology provided herein,experiments were conducted to test the detection of analyte (e.g., H₂S)by an LC sensor comprising various metal salts. In particular, data werecollected in experiments using metal salts other than lead perchlorate.

“Open-faced” LC sensors were prepared as described for the microfluidiccells of Example 1 except the cells did not comprise the top cover.Sensors were exposed to H₂S using the exposure system as described aboveand the response times were recorded as a function of exposure tovarious concentration of H₂S (Table 5). For these experiments, theresponse time was defined as the change in LC alignment that wasvisually observed by a change from dark to bright when viewed throughcrossed polarizers.

TABLE 5 Detection of H₂S by LC sensors comprising metal salts H₂SConcentration Response Time Metal Salt (ppm) (min) Lead Perchlorate 1 3Lead Perchlorate 0.1 25 Zinc Perchlorate 1 1 Zinc Perchlorate 0.1 10Manganese Perchlorate 1 1.5 Chromium Perchlorate 1 7 Indium Perchlorate1 5 Gallium Perchlorate 1 5

These data indicate that the choice of metal salt provides for tuningthe performance (e.g., sensitivity, response time, etc.) of thedosimeter. Further, one of skill in the art would realize that responsescould be further tuned by mixing metal salts, or, more generally, byadjusting the surface functionalities.

Example 12 Direct Detection of NO₂

During the development of embodiments of the technology, “open-faced” LCsensors were fabricated using a micropillar surface as described abovethat was coated with titanium and gold. The gold surface wasfunctionalized with 4-Aminothiophenol (ATP), whose amine group reactsirreversibly with NO₂. The LC thin film layered over the ATP for thissensor was GCB. When the open-faced sensors were exposed to 2 ppm NO₂, aresponse (defined in this experiment as a change from dark to bright asobserved visually through crossed polarizers) was observed withinapproximately ten minutes.

In some embodiments of NO₂ sensors (e.g., as described above), NO₂ at 20ppb is detected by first exposing the sensor to the target gas thenadding liquid crystal to read the sensor. In contrast, the embodiment ofthe device tested in these experiments is a sensor that is fabricatedwith the liquid crystal in place so that the LC realignment occurringupon exposure to NO₂ is directly observed without further additions.These data further indicate that the sensor designs can be modified toprovide performance characteristics suitable for widely varyingapplications.

All publications and patents mentioned in the above specification areherein incorporated by reference in their entirety for all purposes.Various modifications and variations of the described compositions,methods, and uses of the technology will be apparent to those skilled inthe art without departing from the scope and spirit of the technology asdescribed. Although the technology has been described in connection withspecific exemplary embodiments, it should be understood that thetechnology as claimed should not be unduly limited to such specificembodiments. Indeed, various modifications of the described modes forcarrying out the technology that are obvious to those skilled in the artor related fields are intended to be within the scope of the followingclaims.

We claim:
 1. A method for detecting an analyte in a gaseous phase, themethod comprising: 1) providing a liquid crystal assay device comprisinga liquid crystal composition and a surface in a channel; 2) exposing theliquid crystal assay device to a sample suspected of comprising ananalyte; 3) contacting the surface with a liquid crystal; and 4)interrogating the liquid crystal assay device to detect the analyte,wherein a change in a property of the liquid crystal composition in theliquid crystal assay device caused by an interaction of the analyte withthe liquid crystal assay device is indicative of the presence of theanalyte.
 2. The method of claim 1 wherein the surface comprises afunctional group and an interaction of the analyte with the functionalgroup causes the change in the property of the liquid crystalcomposition.
 3. The method of claim 1 wherein the functional group isspecific for the analyte.
 4. The method of claim 1 wherein the surfacecomprises a substrate of glass, silicon, or gold.
 5. The method of claim1 wherein the functional group is 4-aminothiophenol.
 6. The method ofclaim 1 wherein the surface is functionalized with a lead salt, a zincsalt, a manganese salt, a chromium salt, an indium salt, a gallium salt,or a mixture thereof.
 7. The method of claim 1 wherein the liquidcrystal is MBBA, MLC-2080, MLC-2081, E7, or a mixture thereof.
 8. Themethod of claim 1 wherein the analyte is H₂S, HCHO, or NO₂.
 9. Themethod of claim 1 wherein the interrogation comprises measuring a changein a property selected from the group consisting of optical anisotropy,magnetic anisotropy, dielectric anisotropy, and phase transitiontemperature.
 10. The method of claim 1 wherein exposing the liquidcrystal assay device to a sample suspected of comprising an analytecauses a phase transition in the liquid crystal composition from a firstphase selected from the group consisting of an isotropic phase, anematic phase, or a smectic phase to a second phase selected from thegroup consisting of an isotropic phase, a nematic phase, and a smecticphase.
 11. The method of claim 1 wherein the liquid crystal compositionundergoes an orientational transition in the presence of the analyte.12. The method of claim 1 further comprising quantifying an analyteconcentration by measuring the brightness of a reacted area.
 13. Themethod of claim 1 further comprising quantifying an analyteconcentration by measuring a length of a reacted channel.
 14. The methodof claim 1 further comprising quantifying an analyte concentration bymeasuring a distance of a birefringent channel from a site of exposureof the liquid crystal assay device to the sample suspected of comprisingthe analyte.
 15. The method of claim 1 wherein the interrogationcomprises measuring a reflection or a transmission of polarized light.16. The method of claim 14 wherein the distance is 1 to 200 mm.
 17. Themethod of claim 1, wherein said change in a property of the liquidcrystal composition is detectable in real-time.
 18. The method of claim1, wherein said the presence of said analyte is detected in real-time.19. The method of claim 1 further comprising exposing the liquid crystalassay device to the analyte prior to exposing the liquid crystal assaydevice to a sample suspected of comprising an analyte.
 20. A method formonitoring a subject's exposure to a toxic gas, the methodcomprising: 1) providing to the subject a dosimeter badge comprising aliquid crystal assay device; 2) measuring a change in a property of aliquid crystal composition in the liquid crystal assay device caused byan interaction of the toxic gas with the liquid crystal composition; and3) reporting an exposure to the toxic gas.
 21. The method of claim 20,wherein said reporting is in real-time.
 22. The method of claim 20wherein the liquid crystal assay device comprises: a) a first surfacecontacting a composition comprising a liquid crystal; b) a secondsurface; and c) a headspace between the composition comprising theliquid crystal and the second surface.
 23. The method of claim 20wherein the liquid crystal assay device comprises a surface in a channeland the method further comprises contacting the surface with a liquidcrystal.